How to find out someone sexuality?

I like this guy and he's a big ladies man. Nearly the girls like him but they all have came to the conclusion that he is gay and doesn't like girls. He hangs around a bunch of girls but the girls thought it was because he liked them but they think he sees him as “girlfriends.” I've been around him and I don't see the gay in him that they are talking about. I'm not sure why but I told another girl I liked him but she told me not to talk to him.

How to find out someone sexuality?
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