Is there any way to know if the guy you're dating is secretly having sex with his female friend?

So the guy I'm dating doesn't consider me to be his girlfriend because he hasn't made the relationship official. He introduces me as just his friend to everyone even though he treats me like a girlfriend. I confronted him about this and said, ""Friends" don't fuck with each other." and his response was "Yes, they do actually."

Long story short, this made me wonder, does this mean he's having sex with his "best female friend" given that he seems to have no problem with having sex with girls he labels as friends? He has told me that this female friend of his is "looking for a guy" and that he has already told her about me. I know that they are still in touch with one another. I have no idea if they had something more going on in the past or not, but I'm concerned about right now. I know that they went to some dinner gala together and he bought her a corsage. He has never told me that they hooked up or anything like that.

Does it look like he's having sex with his "best friend" or am I being paranoid?
Updates:
+1 y
He rarely talks about her and it's not like he's calling her or she's calling him when I'm around. He's very affectionate, caring, and courteous towards me so I know that he does have deep feelings for me.
Is there any way to know if the guy you're dating is secretly having sex with his female friend?
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