I think you are better off assuming they are cheating if something doesn't feel right to you. In the end whether it turns out you are right or wrong it will not lead to you being happy in the relationship. The main reason someone suspects their mate is cheating is because of (a change of behavior they have towards us).
It could be they are spending less time with you, not being phyiscally intimate with you, fighting over the slightest of things, showing a lack of consideration for your feelings, Not keeping their word about being where they said they were going to be, seem happier leaving you behind than when they arrive home, there is a sense of joy when they talk to friends or family on the phone that seems to be missing when they talk to you...etc Finding out they are cheating would give us the "aha! That explains it!"
However if we found out they were NOT cheating and all of the above behaviors remained present we would not jump for joy or suddendly become happy about the relationship!
Ultimately finding out our mate is cheating just gives many of us (permission) to end a relationship (we are unhappy in). It's kind of sad that many of us need to be cheated on or verbally/physically abused to give ourself persmission to walk away from a "toxic" or unhappy relationship. I say if a person does not like the way they are being treated, feels neglected, or not having their emotional/physcial needs met, then that is enough to walk away whether you find out your mate is cheating or not. You don't have to wait for the (worst thing) to happen before you can walk away from (an unhappy relationship). "Smelling the smoke and leaving is better than sticking around to see if there is fire."