recognize (someone)
To "recognize" someone means to know who they are when you see them. You can recognize a friend or an acquaintance, if they look different than usual or you didn't see them very clearly.
Joe's gotten so thin, I almost didn't recognize him.
You can also recognize a person you don't know, if they're famous or you've seen their picture somewhere.
I recognize you from somewhere. You were on American Idol, weren't you?
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