You don't - that is an invasion of privacy and just plain wrong unless you are told to of course. You obviously have trust issues towards your spouse and should just confront them if there is any suspicion of infidelity. If you think they are sneaking around what is it going to solve if you are sneaking around trying to find out. Two wrongs don't make a right nor do they solve anything. Communication is in order here and you will have to decide if this is worth staying in.