He will begin avoiding you and lying about where he is and who he is with. You will also know when he does not let you get access to his phone like he did before and walks outside all the time to answer his phone.
If someone is cheating on you, you will feel it in your heart if you really love this person. That person has lost all respect, consideration and love for you and you should end this relationship. You are worth loving and having someone who loves you and respects you!
If you suspect he's cheating, have a heart to heart conversation. There's no need to disrespect or diminish what you shared , if his heart has moved on than the relationship with you has run it's course. Always maintain your self respect... A cheater is always a liar and most likely he will do it again and again....there is no joy or love to be found in that kind of a man...you deserve more. I always say.. "The partners we choose, are a reflection of our self worth" you are worth so much more.