For me it will be several things that I find sexy or attractive about a woman, more times than not it will be her hair. What I always do is get to know them, I may not ever mention their hair in the beginning because i'd like to feel them out. After awhile which could range from a couple of week to month, once we're both comfortable with one another and sexual type topics have been discussed, i'll mention that I have a fetish and when asked what kind of fetish, my reply is "it's a secret" and then begin to throw several different fetishes like a foot, hand, cake, breast, etc just to see their reaction, some may reply with an "Ewww", "OMG", "REALLY!?" However the ones that seem like these fetishes are no big deal, boom, then you'll know who may be game for yours. May not be easy for most but was pretty simple for me.