When it comes to trying to figure out if a player is in a relationship, there are really only a few surefire ways. The first and most obvious way is if they introduce you to their significant other or talk about them frequently. If they never mention anyone in their life romantically, it's probably because they're not currently seeing anyone.
Another way to tell if someone is in a relationship is by how much time they're willing to spend with you. If they're always busy and seem disinterested in hanging out, it could be because they're already devoting all their time to someone else. On the other hand, if they go out of their way to spend time with you, it could be a sign that they're interested in something more than just friendship.
The best way to know for sure whether or not someone is in a relationship is to ask them directly. If they give you evasive answers or try to change the subject, chances are they're hiding something from you. However, if they're open and honest about their relationship status, then you can be pretty confident that they're telling the truth.