If you believe she is cheating, dump her.
Apparently you think she is cheating even though you have no proof. Why would you want to date someone you don't trust? It makes no sense.
The only reason you want to "catch her" cheating is for ego purposes. You want to see her reaction when you say, "Got yah!", show her how "smart" you are.
If cheating on you is a truly a "deal breaker" and you are with someone you believe in your heart is not trustworthy then you're better off saving yourself some time and move on. Playing private detective especially in a non-married situation offers little benefit to you.