Tbh, this is a silly topic, deserving a silly answer....
you never know when you pass... because people may just be being polite, or you may pass. how do you find out?
'hey, what sex do you think i am?' doesnt quite work does it?
only way to find out is out yourself, then your not passing

So best solution is rock on and stop caring. i did, and my life is 100000% smoother and easier.
Sure its a big deal, and it dominates your mind for a bit. but it does go away.
when do you stop passing?
when you ARE that. you really cant pass as a girl when you are one. and when you are one, you pass!
We see features most dont, and look at a random face, mostly women, and youll see those features you think stop you passing...
most people only take cursory note of gender markers...
no stubble, no shaddow, long hair, in womens clothes, most will go female
beard, instant guy....
the rest is just how you look....
R >