Idiom for something that a person does that further bolsters your belief about something

Example: I had by doubts about someone that he is a kleptomaniac and the other day I saw him stealing something and that action further strengthens my belief that he is a kleptomaniac. Any idiom for this type?

When you saw him stealing something, it entrenched your belief about his nature/behaviour.

Or it lent credence to your belief about his nature/behaviour.

To lend credence/plausibility to something means to make something more credible or probable.

I think you could also use reaffirm.

There are a number of ways to say this that aren't really idioms as such, just common and typical, bordering on cliché:

• I was proved right about [person] when they [confirmed my suspicions].

• [person] confirmed my suspicions [about their nature] when they [...].

• I had long suspected that [person] was [a kleptomaniac], ...

  • so I was not at all surprised when [I saw him pocketing something in the local supermarket].
  • a belief reinforced when [etc]

Your original description is fairly idiomatic (i.e. fluent English) already.

One idiom that is close to what you are asking for, but doesn't presume you already had a suspicion about the person (or even that it's a bad thing you suspect, though it usually is) is to say

• [person] *really showed [his/her/their/...] true colours today

which originates from the sense of someone pretending to support one side in a battle, suddenly revealing themselves to actually belong to the other team.

How about the noun phrase proof positive? From M-W:

proof positive: something which definitely shows that something else is true or correct : definite proof

Proof positive captures your "something that a person does that further bolsters your belief about something" -- that you saw the person in question stealing something was proof positive of your belief he is a kleptomaniac.

Addendum: Based on the exchange below with @OwenReynolds, I'm going to offer up another answer, which is in fact an idiom: caught red-handed. From The Free Dictionary:

caught red-handed: Seen or apprehended in the act of doing something, especially something illegal or nefarious.

If the OP believes another person is a kleptomaniac and catches that person in the act of stealing, that would certainly "further bolster" that belief but not prove it.

Update: I answered this question on the premise that only an idiom would be acceptable. That assumption is borne out by the question title and the single tag used.

However, half of the answers here, including the one now accepted, do not provide idioms, but simple words or everyday phrases.

I would not have answered in the way I did had it been apparent an idiom was not actually required.

If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.

From the Wikipedia page on the "duck test":

The duck test is a form of abductive reasoning. This is its usual expression:

If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.

The test implies that a person can identify an unknown subject by observing that subject's habitual characteristics. It is sometimes used to counter abstruse arguments that something is not what it appears to be.

This idiom is also commonly shortened to just if it looks like a duck …

There are of course variations on it. For instance, Douglas Adams (this is mentioned in the same link as above) humorously said the following:

If it looks like a duck, and quacks like a duck, we have at least to consider the possibility that we have a small aquatic bird of the family anatidae on our hands.