Give them an easy opening point, or kinda set the conversation up for THAT kind of talk.
I would never approach one of my guy friends for a FWB situation, and I'd probably slap one of my guy friends if he came straight out and said "hey I wanna bang you but no strings attached, k?"
HOWEVER, having said this, some conversations will guide you down that path with little effort required. If you can somehow bring the topic of sex to the conversation, in a non-threatening, casual kind of way, you'll soon find that the conversation either gets really really weird, or really really dirty. Either way, you'll get an idea of whether or not to bother.
If things do start getting dirty, egg it on! Don't be sleazy, just be flirty and maybe suggest that the two of you should explore having some fun, if they're up for it. They still might say no, but if you get the friend balance correct, you can back off as soon as you see them hesitate and be like, "haha well can you blame me for trying? You get a man all worked up, what do you expect? *Winky face*" - keep it playful, laugh it off, don't sulk or get upset if she turns you down.
Plus, there are other factors involved. There's definitely an element of "being in the right place at the right time", because the ideal FWB partner is looking for the same thing you are - No Strings Attached Sex, a little bit of friendship, nothing serious. People just out of long term relationships are often good candidates because they're not looking for long term stuff and are usually still a bit preoccupied with their ex. Double edged sword, though, they might be looking to jump straight back into that full on relationship with the next guy who crosses their path.
Like everyone has said, you will always run the risk of them/you developing stronger feelings, but that's the risk when you play the game.
My current FWB situation started with an innocuous sex talk over msn that turned dirty. Lots of fun, but know what you're getting yourself into. Make sure you stick to the rules - and establish them BEFORE you hook up. Communication, honesty and RESPECT are key, here. Don't be a jerk, especially if she develops feelings. Know that this kind of thing always has an expiry date, and make an effort to keep the friendship primarily about the friendship, and the sex as just an added bonus.