If the person looks like a man, you can say "I think he's a man." (For example, you're looking at a poor quality video of a crime being committed, and you're trying to identify the perpetrator, who appears to be a man.)
If the person looks like a woman, but you suspect that person is a man, you can say "I think she's a man." (For example, you're with a friend in a Bangkok nightclub, and a beautiful woman approaches you. But you notice an Adam's apple and a bit of stubble.)
But depending on the circumstance, you could offend some people. For example, some transgender people may object to such statements.
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