Body Odor, how will you know?

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Everybody knows each person has his/her own odor. And there's no exception that when we sweat, we don't smell nice. But I'm wondering how will a person distinguish that what he's smelling is already a "bad odor" or not? I mean so far my own distinction of a person's "Bad body Odor" if...(excuse me) someone smell like a rotten onion...(excuse me again) I think this is my own distinction. What do you guys think?