How to know if my friend has a crush or something with my current girlfriend?

Anonymous
So I got with this girl that I met 1 year ago, we did get together after only one month meeting each other but I had to end it in less than a month in that relationship and now we got back together again… This girl is my best friend’s female friend for over 10 years now, he is the one who introduced me to her. Before it even crossed my mind that I find her attractive and more than a friend I kept asking him if he is interested in her but he said she is only his friend, when I got with her the first time he was one of the many reasons I had to break up with her over, he at point even said she was coming onto him too (Which if I am being honest I don’t belive because I know every detail of when she started flirting with me and got really attracted to me).
The thing that confused and still confuses me is I think he has some interest in his best female friend who is my girlfriend now, but I wanna know some telling signs if you can guys!
How to know if my friend has a crush or something with my current girlfriend?
1 Opinion