Knowing quite a few friends and parents of friends who have come out. The likely hood is that they already know, if not completely then on some level.
You've kind of got to ask yourself what the worst thing they can do is. Chuck you out or say they hate you. Are they those kind of parents? If you think it's possible that they might chuck you out then move out before you tell them if you can.
Mostly even if parents have a problem with it they'll just be glad you told them. I normally find the easiest way to get my folks attention is to to them throughout the day with something like 'Hay there's something I want to ask your guys advice on something do you both have some time this evening?'