If there is someone who has your attention that you start to talk about, he will most likely close off immediately.
He will never ask you to not talk about him or let you know that he has a problem with you having someone in your life but you can see in his behavior that he’s not okay with it.
While you’re talking about another man, he is thinking about his lost chance and the ways he can make it better.
He’s hurting at that moment but still tries to act like nothing is wrong, because it’s his, and only his, fault that you don’t know about his feelings.
You can do this as a sort of test, if he’s a hard one to read. Start talking about a guy who approached you at a club and how he had all the right manners and moves and how you can’t wait to get to know him better.
If his mood instantly changes and he tries to change the subject while stuttering about how happy he is for you, you got him.
But don’t do this unless you’re ready to make a move on him. It’ll only mess with his feelings and confidence and that’s not the right thing to do.