LOL at some of these responses...the OP is about to make the biggest investment of his life (marriage), and you guys are telling him he fucked up by doing some research beforehand. LOLOL you wouldn't buy a car without a test drive, and you most certainly would not sign a contract (what marriage essentially is) entitling someone to half your net worth and future assets without doing, as he put it, "due diligence."
Trust is an earned priviledge, not some unconditional right that anyone is entitled to.
OP, have you considered taking a quick glance at the house's mailbox and seeing who the mail is addressed to? Shouldn't take more than 5 seconds. As a back-up, take some random flyers with you (from anywhere) and just say you're distributing them to local neighborhoods in case you get caught.
Unfortunately, however, just because you find out who lives at this house, it really doesn't give you much proof as to anything that may or may not be going on there. Say it is a male...can you prove she actually went in his house? How accurate is this GPS? Does it show you that she parked in the driveway, or just on the street? There could be a million possible explanations for this scenario...
If she found out you did this, you would most definitely lose her. From her perspective, she will see it as a breech of trust, an invasion of privacy, and a power tactic (as in you feel that you own her, and have a right to know where she is at all times - not saying this is the case, but this is how she will see it). In other words, you probably don't want to tell her about this. If you think she's cheating, you will need to find some other form of evidence to support your theory.
They never do.
Moral of the story: If you believe that she's cheating on you, find more evidence before you confront her about it. But, for the love of God, don't marry her until all of your questions are answered.