There is no direct way of knowing if someone has been looking at your iPhone.
But there some indirect way of knowing. For example, he could view apps in a specific order, then if you double tap the home button to see multi tasking screen, the apps will be sorted in the order they were lasts used, if the order he left the apps is different, he might recognise it.
Same if he leaves mail or messages app on a specific message.
I'm sorry you're going through this, it sounds like you have more marital problems than technical iPhone issues.