There are 2 ways to do this Gamma Ray. First, i would suggest that you don't just her straightforward accuse her unless you have real evidence. Keep your dignity intact.
Second - if she is dating someone else, then you need to take a call, whether YOU want to pull the plug. If you decide to pull the plug, then tell her your reasons. If you want to wait around, do that till something precipitates. I will ask you again - are you in love with her? How deep is your relationship with her?