my ex is one of my best friends. does he do this in front of you, or when he thinks you're not looking? don't break up with him if you're just suspicious... if you can probe he's cheating then go ahead, but like I said, my ex is my best friend, we hug and joke around, but we don't do anything more.
Do you feel uncomfortable about this? Is this putting a crack in your relationship? If so, you need to talk to him about it. If he doesn't understand and you can't work something out then this could mean there is something else wrong in your relationship, maybe a lack of respect on his part or trust issues with you. If he's not willing to work with you on it, it may meant that you need to break up with him.