How do I know if my boyfriend is cheating on me (again)

Yet again he has gone through my phone when I was in the other room watching tv. Right before falling asleep last night he asked me who AJ was then when did I call his coworker. AJ is a girl & I called his coworker in front of him! He didn't remember it that way of course. The last time he suspected me of cheating he was having multiple online relationships. The other day he left his phone with me on accident. I did not go through it. I thought we were past that. Obviously not. I've learned from friends relationships & my own past ones that when a partner accuses another of cheating normally it's the originally accuser who is not being faithful. So I asked him last night did you go through my phone & he says NO! so how did he get the information that he remembered incorrectly?? How can I know if he's being unfaithful to me ( again ) ?