If you were out of town and when you got back you found a girl's phone number on your boyfriend's cell phone and he said he didn't even know it was there how do you get to the truth?

He could be telling the truth, but if this were to happen again phone the #! Ask who it is, and ask this person why they are calling. Don't accuse them of anything in case it is work related, but if she is seeing your boyfriend she will have great joy in telling you on the phone. Your boyfriend has no control over anyone calling him on his cell, and since he didn't erase the call off his cell phone he doesn't appear he is hiding anything unless he doesn't know how to erase his calls. I think by now he gets the idea that you are onto him. Don't let this one call ruin your relationship because sometimes some people do tell the truth. Just take it slow and easy and keep your eyes and ears wide open. Good luck Marcy