So the big question is how do you find out if he's keeping any secrets if you don't invade his privacy?
the answer is... you can't. You are going to have to cross the line for confirmation and personally, I don't see anything wrong with you confirming the loyalty and the trustworthiness of the one you entrust with your heart.

We confirm everything else; hotel reservation, dinner reservation, payments etc.
If there is nothing there, all the better. Confirmation simply proves or disproves your beliefs, one way or the other. I have found that the ones who rely heavily on the " privacy " position, are often the ones with the most to hide.

having said that, I also know that some of you won't cross that line because it opens you up to the same and willing to take that risk. why? Because some of you have your own secrets to keep.