Hmf. this is a tough one.
Try and feel them out, maybe casually bring up a famous transgender person and see what they say about them?
or maybe say a friend is going through a issue similar to yours or something like that... In terms of coming out fully to the, i have no idea.
You have to hope for the best and prepare for the worst i suppose, even the most supportive parents sometimes have bad reactions to it. and sometimes even the most strictest parents will surprise you with how supportive they are. its all about feeling them out and deciding whats best for your situation.
Hope everything works out for you.