If you're interested in signing up for a league that is based on age groups (e.g., under 16, under 18, etc.), then you may have to submit a birth certificate, because they care about whether you are age-eligible. That could out you.
It's pretty uncommon for an ordinary local league to have steroid testing.
A lot depends on what kind of league it is (public, private), why they segregate by sex, etc. You could sign up for a guy's league as stealth, but in some circumstances, they can kick you out if they find out the truth. It depends. Chances are you may want to let someone know higher up in adminstrating the league, while still staying stealth with the team itself. It may be important for the coach to know, though. If you get hurt, the coach is typically the one who has to be first aid certified and would make sure you get the medical care you need.
Maybe your parents can call the league and ask and do it anonymously, so if they get unfavorable answers, they haven't ruined your chances of taking a different approach?