How does site like this one know if your refferals are legit?

I would think that they could easily check the IP of your computer. That would be one way they could check.

believe me all programs have ways to check it, but they aren't going to tell you how they check because then people would try to find a way around it and cheat. Considering this site pays, I'd say you want to follow their rules and not try to take advantage of them, if you try to abuse the system eventually you will get caught!