Meaning of "How'd you know?": "would", "did", or "do"?

Solution 1:

This question can be satisfactorily answered only if a specific context is provided. For example:

A: I broke up with my boyfriend because he was cheating on me.
B: How'dja know? = How did you know? because A's sentence is past tense.

C: Most Americans own AK-47s.
D: How d'you know? = How do you know? because C's sentence is present tense.

E: I'll shoot my girlfriend if she cheats on me.
F: How'd you know? = How would you know? because E's sentence is a future condition.

Notice that I changed the contracted forms. I did that because the pronunciation of the spoken English sentences would change. That doesn't mean that the written form would change. Not everyone can or wants to or bothers to provide readers with an accurate phonetic transcription of how they'd say such sentences.

Another point: The E-F dialog is the least likely one to use "how'd you". I think most native speakers would say How would you know? I know that I would.

Ambiguous sentences are a constant problem in all languages. Listeners and readers have to interpret them as best they can. However, when you're actively engaged in a conversation, you can always ask the speaker to explain. When you're reading, you have to use clues in the piece of discourse you're reading to figure things out. It's usually not terribly difficult, but there's no guarantee that you'll always get it right, despite the confidence that so many writers have that readers can always read their minds.