Cheaters cheat themselves

After having decided where you stand, you may want to find out if your suspicions are actually facts. So how can you find out?

Now something you could do would be to read up on personality traits which are common in people who cheat, but this would only be one more indicator and not proof. There are countless options (i.e. you could hire a private eye), but the most simple thing to do is just to wait and watch carefully. While I had an anonymous tipster I suspected cheating for a long time and this even though she was cheating from the beginning. She was giving herself away.
At some point a cheater will stumble over their own lies or attempts to hide their cheating. It's just a matter of time. She was contradicting herself and things were just illogical. Things she said she did are not things she would have done or she even said she would never and did never do things in way in which we actually did them together.
The problem someone who cheats faces is that they need to create an alternative reality and in doing so they will get lost in that reality at some point. They probably start to believe parts of it themselves because (most) people's brains simply can't handle this and so false memories are created and they might even start telling you that they are like this or that when they aren't. Cheaters cheat themselves and so give themself away and deprive themself of real happiness.

So just be yourself and watch your partner carefully.

P.S. Just the fact that you are suspecting probably means you are on to something. Trust your senses! Often people try to make you doubt them, telling you just imagined something or similar. The truth however is that you are more often right than wrong when you heard or saw or smelled something. Humans probably wouldn't have gotten very far if their senses were as bad as some people try to make you believe.