The Forbes article “How to Track a Cheating Spouse” is one way to find out if your mate is being faithful. Being that I am all about communication, I prefer the direct communication route. If you ask your spouse to their face if they are sleeping with someone else, chances are you will be able to tell from their reaction, even if they lie straight to your face.
But, I guess if you try the direct communication and they lie, you can do as this article suggests and use technology to track down their activities. Now we not only have phone logs and kept text-message conversations, but we even have GPS to track if people are where they said they would be.
So, if direct communication fails . . .