All of the above answers are great. I'd like to add something I've done in the past. This works best if you two have watched or will watch a porno together. Find a video (magazines are ok, but not preferred) that has a guy getting fisted. Check out her reaction... if she's not horribly disgusted tell her that interests you. If she does seem very put off by it then you can decide if you want to explain to her that you think it'd feel good, or expose her to it more....
On the other hand.. if you find yourselves in a particularly hot love-making session, there's nothing wrong with saying "give me more baby, that feels so good!" or something on that order. Women do this, so why can't a man?
Both of these really depend on your relationship though. Just my suggestions :)