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

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.

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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 …

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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.

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.

That tears it

The internet says it means merely "ruined". But as a metaphor it means your trust or confidence or good opinion is like a garment. Previous incidents have ripped it a little, but this final thing has torn it in half. "Well that tears it -- he can't be trusted" only makes sense if there were some suspicious incidents, but the group still trusted him somewhat.

It's obviously similar to "the last straw" or "the final nail". But those are about tolerating known bad behavior. You've known they're a thief for quite a while before one act can be the final straw.

But it's not a perfect fit. "That tears it" means your opinion was confirmed, not merely bolstered. It also only applies to something bad. And it applies to any belief, not only one about a person: "that tears it -- I could live with the other stuff, but we're not going to Jersey if we can't get a room by the beach". It also implies some action on your part. "We suspected he was a thief, this tears it. You can't invite him to any more parties" would seem odd without the proposed solution. Our trust in him as a guest has been torn.

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.

For the specific example, I'd use bolster.

bolster: to support with or as if with a bolster : REINFORCE

I had by doubts about someone that he is a kleptomaniac and the other day I saw him stealing something, which bolsters my belief that he is a kleptomaniac.

A leopard can't/doesn't change it's spots:

Something you say that means a person's character, especially if it is bad, will not change, even if they pretend that it will.

[Cambridge English dictionary]

"Blood will out" (note that this could be considered racist, as you are implying that your suspicions were informed by his specific nature or heritage)

"It just goes to show you"

"I thought I smelled something fishy"

"I had my eye one him (and wouldn't you know it)"

"The apple doesn't fall far from the tree" (if you knew that one or more of his parents were kleptomaniacs)

"...and I was right all along."

All of these are idomatic expressions in the sense that they likely make sense to a native speaker of English because of prior experience, but generally cannot be translated or understood literally by the meanings of the words themselves.

Idiomatic: "using, containing, or denoting expressions that are natural to a native speaker."