You'd be amazed. I have seen people get caught by leaving their e-mail open while their partner is around. I have heard of one guy sending an e-mail to his mistress asking her to marry him. As far as he knew his GF trusted him and wouldnt look at his e-mails. You can guess how that went ........!
To answer the OP if you dont trust the person dont get married but if you want to be with them and you get caught spying and they turn out to be faithfull you are putting everything at risk.