6. See if he avoids standing close to anyone other than his close friends
Guys who already have a special someone tend to distance themselves from the opposite sex. Some guys will naturally avoid physical contact with women even if they are single, but this is still a good sign to look for. You might see him hanging out with a group of his guy friends and avoiding standing close to any women who come over to the group. This could very well mean he isn’t looking to hook up with anyone since he’s already in a relationship.