Should i take her back?

So me and my girl broke up. A week later she went on a date with this guy. Well we hungout to maybe try to make it work and i saw a hickey on her neck, when i asked her she said it was a bruise. Later we hungout again and she finally admitted it was a hickey. She now wants to get back together should i take her back? I have a feeling she had with the guy too but how do i find out? If i ask her i dont think she will tell me the truth