i dont think its easy to tell if a girl will like it unless you have discovered something that one might consider kinky. If so, regardless of the kink, it is rather easy. Just bring up the kink sometimes when it is common - maybe, when u have sex, and if she responds positively, approach her about it. How soon you approach the ? totally depends on her response.
Wait until you see a physical reaction before you ask her to do it, ex) if u chat about it as fantasy and her pussy is wet soon afterwards (1-2 min), then you know she is thinking about it.