You don't. You just don't. I base my trust (which has been broken before, and has taken a long time to come back!) on the fact that my partner has given so much out of 'guilt', so much money and effort and time into making sure we healed properly - and he simply isn't the kind of man who would do that just to save 'face', he doesn't sugar coat things. With the state I was in whilst we were healing from his mistake, he could have easily just finished things, those breakdowns were never fun for him. Now, I know that he respects our relationship. So whilst I'll never say 100% that I'm sure he won't cheat on me, I do feel confident that if he did, he wouldn't be able to live with himself after everything we've already been through; he would finish it, maybe under the guise of another reason, or he would admit it .
But you simply can't know, and that's why it's horrible but also at the same time why it's necessary. It's a bit like religion, or Peter Pan's fairies, sometimes; if you don't believe in it, it doesn't exist. So have faith in it and give it a go.
Don't look through your partner's phone messages. On the same vein that you should be creating a relationship in which one A wouldn't want to cheat and B couldn't continue after cheating, you also are trying to form a relationship in which underhand, contradictory things such as reading messages or Facebook conversations just aren't necessary. I went through a big phase of doing that - it was 'rewarding' once, in that this is how I found out about things, so my logic was how can I stop when each time I look I find something? But I just have to put it aside. If you're in the relationship, you're in it - don't disrespect your own decision to be together by doing this paranoid, distrustful things.
If I took the question to be asking for a different thing haha - avoidance, anger issues i.e snaps at simple questions, compensates, gets fearful when you borrow laptop/phone, has a 'regular' thing like visiting a friend you don't know some weekends.