"find" and "see" are fine. "learn" is possible, but it suggests that he (or someone) told you, rather than it being apparent from his appearance or behaviour. "know" doesn't work in that sentence.
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seagullwe cannot use "know"Correct. The usual verb that references the moment of "starting to know" or "coming to know" is "find out", but "find" without "out" is also possible in the context of your sentence. "know" can't reference that moment between when you didn't know and when you knew.