How will can I tell if my girlfriend is lying about not doing stuff with a stranger at a bar?

I found a picture of my girl with a stranger last weekend and they had there arms around each other at a bar if I ask her what else happened how will I know if she's lying to me about anything cause I know she cheated already but what else happened cause she fought with me the whole time she was gone went without me and meet talked to me when she did this and still hasn't so when I ask her about it all how wil I know she is lying or not telling me about something she did?

Pictures can be deceiving. Unless you were there to see the context it was taken in, you'll probably never know the whole story. The problem here is not the picture, it's her hanging out in bars. There is nothing wrong with either of you having a few drinks with your friends and leaving together but if either of you are hanging out in bars without the other, that will destroy a relationship. Trust is a principal ingredient and this one's obviously missing that ingredient. It might be wise for you to pass on the party girls and find a homebody for the next one.