ID:260780
 
var/list/favorites=param2list(world.favorites(usr.key))
if("Blahblah.game" in favorites)
usr << "You're awesome! Thanks for supporting this game!"
You can always do this yourself, I remember seeing a library/demo of this before.
In response to Jeff8500 (#1)
I made a library for this a while ago that I never uploaded. I just uploaded it now:

http://www.byond.com/developer/Flame48/GetFavoriteGames