There are many ways to tell this.

firstly her anger. she may not be as angry in a argument as she use to when you first met and if you question where or what she has done on that day she may start to think its a heated discussion even though its not.

The main thing if your partner is cheating, if you start a long discussion, they WILL eventually start to trip up, meaning they will start trying to lie even though it may not relate in any way to another lie they have said in the pasy.

There are several ways to tell but i mainly concentrate on these shown above.