All of these come from personal experience from having been cheated on in a 6 year relationship. These signs are not 100% signs that he is cheating- but were indicators in my case.
1. He is always working late these days, or is "busy"
If you used to see your man all the time, but all of a sudden he is too busy to spend time with you (and there was no significant project at work, or promotion etc) then its time to open your eyes. He may be spending his time with someone else.
2. He takes his phone with him everywhere he goes
If your man takes his phone to the toilet, shower, and other places that he usually doesn't then its a good sign he may be keeping something or someone from you. Pay attention to whether he has put a password on his phone (which he hasn't given you the code to), or if he started silencing his phone, and taking conversations in the other room.
3. He stops initiating sex
SIMPLE. If you used to have sex once a week, and now you're lucky if you get it once a month... something might be up. After all, if he loves you then he should want to be affectionate towards you and treat you like a woman.