My sister (who is now deceased) married someone from Colombia in 1993. He paid her $2500.00 to marry him so he could get his green card. How do you find out if he actually had a valid green card before marriage or if he got it after the marriage?

If he paid her to marry him he obviously didn't have it. But if it's 15 years ago I don't think USCIS (or INS as it used to be called) will take the time to investigate it... especially since your sister isn't around anymore to be questioned.

It's probably best to just let it go.