I usually park my domains that have some type of link popularity... this way you can determine what type... if any, traffic it receives. If it's getting a good amount of traffic - you should earn revenue through clicks.
I've also parked domains with 0 link popularity, but with "highly searched" keywords that produce revenue rather nicely as well - an example: FreeEcards.us - killer combo... and I would have never imagined this would earn so well because of the "triple e"/ccTLD combo. It's in my top 5 earners over at Sedo... and I've done NO promo at all on that name. Much of the time it's trial and error!
So it either has traffic, or keywords that :kickass:
Good Luck
Jennifer