How can I tell if he's telling the truth?

there's this guy I like and he says he has a girlfriend but I don't believe him like we stare at each other for a solid 10 secs without saying a word and when I'm around he always makes sexual jokes. Should I keep asking him if he actually does have a girlfriend like should I ask him what her name is where they met how long they have been together?
How can I tell if he's telling the truth?
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