It's hard to say 100% how to tell.But first sign is your "6th sense" that usually tells U that something is wrong....Other signs can be that she has "secret"phone calls that she hides from u...etc...The best thing is talk to straight to her even though that can be not sufficient cause she would probably lie anyway even it's true....I wish u good luck but as I already mentioned "the 6th sense" works..It worked for me and then I found bunch of evidence of my ex cheating on me...That's why he 's ex now....