Well You Could Try These....My Not Help U But Atleast I Tryed...

First thing u could do is  look in her eyes and ask her "do u love me?" on the way she ansters you will probley no if shes lying or not...(worked for my m8 :P)

The second thing i no is that she will allways be out or needing to see her friends more offen... or maybe she would like never look in to your eyes or always look away from u and never talk about her day and stuff..

well i hope this help and if your gf is really cheating on u the omfg!!! lol well good luck