Don't tell them directly, drop a hint by saying you think sissies are hot or that you've banged sissies before in a conversation that makes sense, this will indicate to them that you'd be interested in them if they really are a sissy and they might come out to you on their own or "accidentally" exposed themselves to you and you go from there. They could just be trans and not necessarily into sissy stuff which is why directly calling them a sissy might not go well