Good way to test the waters! Anyway, if they seem supportive, i say come out when you are ready. Think about how you feel coming out, do you feel anxious and worried, or do you feel proud and like you want to share your gender with others? Think about this first. If you feel like you are ready to say it, then go ahead, it sounds like your parents are supportive. I recommend doing it in a way where everyone (you and your parents) is calm and can talk about it. Maybe a family meeting of sorts. But something like that.