how do you get the answer without setting yourself up

My question is how do you ask your partner about something you found when you were snooping around thier stuff?

I looked at my boyfriends phone the other night and saw two girls names the recent call history.  How do I ask him who they are without giving away that i went through is phone?

I dont have any passwords to his emails or his phone bill or any other means of finding out.

It is a touch subject to bring up, beacuse when we first got together he was seeing other women and i called him on it and gave him hell. and it was like pulling teeth to get any truth from him at all. I still find it hard to believe that he tells me the whole truth and not just half truths.  Its been two and half years now and we live together and work together.  I dont want him to think that i can never trust him again or for that matter that he could never trust ME again since i was snooping.

What do I do, each minute that goes by I assume the worst and start to have distain and pull away from him.

help please