How can I reliably and accurately identify the passive voice in writing or speech?

How can I reliably and accurately identify the passive voice in writing or speech? I'm not interested in advice about whether or not to use it yet... I just want to know for sure what it is, so that I don't look as stupid as these people.

If a clause has all of the following, then it is in the passive voice:

• A form of an auxiliary verb (usually be or get)
• The past participle of a transitive verb
• No direct object
• The subject of the verb phrase is the entity undergoing an action or having its state changed

Example: The documents were printed.

Optionally, the agent is expressed in a prepositional phrase with by: The documents were printed by the printer.

There are some exceptions; though, generally speaking, if a given clause meets all the above conditions, then it is certainly passive voice. The Wikipedia article about the English passive voice has a pretty complete coverage, detailing all cases of English passive voice, but the major exceptions are these:

• A passive clause may have a direct object in the case of ditransitive verbs; when the indirect object is promoted to subject, the direct object remains. (Someone gave Mary the documents becomes Mary was given the documents.)
• In concealed passives, the verb form is a gerund-participle and has no auxiliary. (Your document needs printing)
• In bare passives, the auxiliary is missing, but these clauses can only be used as modifiers (With the document printed, Mary could hand in her paper), or in special syntactic constructs like newspaper headlines (Document printed by printer).
• Some related forms, the passival (The document is printing) and middle voice (These documents print well), may be considered to be kinds of passive voice.

Passive voice is a construction where the object of a transitive verb is moved into the subject position, and the subject is optionally moved into a prepositional phrase. In English, the passive can always be identified by to be + past participle. Some examples:

Active: Kim hits the ball.
Passive: The ball is hit by Kim.

Active: Grandma baked a cake.
Passive: The cake was baked by Grandma.

Active: Mr. Henry bought the painting for six million dollars.
Passive: The painting was bought for six million dollars.

Note the following:

• All of these sentences contain a form of the verb to be followed immediately by a past participle. Without that telltale, it's not a passive sentence.
• The prepositional phrase with by is optional, as in the last sentence. However, a sentence that identifies the actor with a phrase beginning with by is usually passive.

Now, to clear up some common misconceptions.

Intransitive verbs are never passive, even if the subject of the verb isn't doing anything. For example, none of the following are passive:

The boy fell down.

Six buildings burned to the ground.

The cake is baking.

All of these sentences have intransitive verbs, which are verbs that do not take an object. The fact that the subject of the verb isn't really "active" in any of these cases does not make these examples of "passive voice". All of the previous are in fact active voice.

Second, passive voice has hardly anything to do with the "focus" of the sentence. For example, the following is not passive:

We all watched John make a brilliant save.

The "focus" of the action here is John, but that's irrelevant to the question of active voice and passive voice. The main verb watched is in the active voice, and John is the object of watched. The passive version of this sentence would be:

John was watched making a brilliant save by all of us.

(Which is an extremely awkward sentence.)

Finally, there are two other constructions sometimes misidentified as passive because they share some syntactic features with the passive voice, but which emphatically are not passive.

The first is the progressive, which consists of to be + present participle. (The present participle always ends in -ing.) For example, none of the following are passive:

Kim is hitting the ball.

Grandma was baking a cake.

Although these contain a form of to be, they aren't passive since they don't contain the past participle.

The second is the perfect, which consists of to have + past participle. For example, none of the following are passive:

Kim has hit the ball.

Grandma had baked a cake.

Although these contain the past participle, they aren't passive since the participle doesn't follow a form of to be.

Grammar Girl had a good podcast on this very topic recently: "Active Voice Versus Passive Voice"

She had a really good definition for passive voice:

What is Passive Voice?
In passive voice, the target of the action gets promoted to the subject position. Instead of saying, "Steve loves Amy," I would say, "Amy is loved by Steve." The subject of the sentence becomes Amy, but she isn't doing anything. Rather, she is just the recipient of Steve's love. The focus of the sentence has changed from Steve to Amy.

I really like her debunking the myth that a sentence is automatically in passive voice if the verb is a "to be" verb. For example the following sentence is definitely in active voice despite what Strunk & White think.

I am holding a pen.

Mass insanity over passive UFOs continues, the Language Log article linked to in the OP, is one of a series by Professor Pullum, co-author of the monumental Cambridge Grammar of the English Language, in which Pullum derides comments in the "news media and political blogosphere" that misattribute the term passive to any construction in which the speaker or writer appears to be attempting to conceal the agent.

Thus, for example, the grammatically-clueless political commentator detects the attempt to avoid agency in both There were mistakes in the planning of the operation and Mistakes were made in the planning of the operation, and assumes that the term passive can be applied to the former because it is correctly applied to the latter.

Of course, there is no reason why the term passive should not evolve to denote any utterance that obfuscates the agent, but for the purpose of this answer I shall stick to a discussion of what grammarians understand by the term.

Before we can identify something, we have to define it. Since dictionaries are the usual place to look for definitions, it is helpful to cite from the entry for passive in the Oxford Dictionary of English Grammar (ODEG), p294-296:

A construction (verb phrase, clause, or sentence) in which the referent of the grammatical subject typically 'undergoes','experiences' or 'receives' the action of the verb (i.e. is its patient).

...

In formal terms, a passive construction contains a form of the passive auxiliary be (or get) combined with a past participle.

So to test whether any given construction is a canonical passive construction, we need to ask:

• Does it contain a form of the be or get auxiliary together with a past participle?

• Does the grammatical subject undergo, experience or receive the action of the verb?

In most cases these tests will be sufficient to determine whether a construction is passive or not. For example, the first test will immediately rule out the There were mistakes construction since it does not contain a past participle. Conversely, Mistakes were made contains a form of the be auxiliary together with the past participle, and the grammatical subject, mistakes, can be said to undergo the action of the verb, to make, so it is passive.

Of course, it is the nature of language that there will be edge or non-canonical cases. @Nohat's answer includes some examples such as bare passives, and links to Wikipedia's article on the passive, which in my opinion is a good overview of the topic.

The Wikipedia article also deals with the issue that appears to be most exercising the setter of the bounty for this question - namely, how to classify constructions such as I am surprised by you, Mary.

For a discussion of this point, it is useful to cite the ODEG once more:

Some passive sentences are ambiguous. Thus The opera house was finished in 1980, out of context, is most likely to have an actional meaning referring to the building activity, namely 'the building work was completed in 1980'. This is called an actional passive. But it could also have a statal meaning, i.e. 'the building was in a finished state in 1980'. This is called a statal passive. In an actional passive the string be + past participal clearly forms a passive construction, whereas in a statal passive be is a copular verb and the -ed/en form is an adjective. The statal passive is sometimes called pseudo-passive or adjectival passive.

So, finally, how do we determine whether constructions such as I am surprised by you are actional passives or statal passives? Well, a reliable classification of decontextualised constructions is not possible since we don't have access to the speaker's communicative intention. But we can speculate, in which case it is useful to return to the second test:

Does the grammatical subject undergo, experience or receive the action of the verb?

If the speaker means something like Everyday I am surprised by you creeping up behind me while doing the dishes, then we have an actional passive because the I is receiving the action of the verb to surprise.

If, conversely, the speaker is making a general statement that he is in a constant state of surprise caused by Mary and the various things she does and says, then we have a statal passive.

Since writing the Mass insanity over passive UFOs on Language Log, Pullum has added another article, whose explicit intention is to teach the grammatically clueless what constitutes a passive construction, and what doesn't: The passive in English.

The English passive structure to be + past participle is theoretically ambiguous. It can describe a passive action (1 The bridge was destroyed) or it can describe a state/condition (2 The bridge was destroyed).

You decide which of the two possibilities is meant either by context or logical judgement.

English can avoid this ambiguity by adding a passive agent (3 The bridge was destroyed by soldiers) or by using the continuous form (4 The bridge was being destroyed when we arrived) or the get-passive (5 The bridge got destroyed).