You really can't, unless you catch them of course!
I have been having an affair with a married person.. If that person is willing to cheat on their spouse, how can I tell if they are cheating on me?
There really isn't a way to know unless you see this person with another person who is not their spouse. Then they aren't only cheating on you but cheating on their spouse more than once. It's kinda like in the movie "The Other Woman". It's a pretty silly movie, but it does show an example of what "cheating on your spouse with multiple partners" may look like.
Well, if he's cheating on his wife with you, then I would easily assume he would cheat on you also. Someone might just come along that he likes better.
Men that cheat on their wife will certainly cheat with anyone who lets them. I think he would cheat on you.
Now your answer is with you - just imagine deeply and decide what you have to do.