Not attracting attention is what I call level 1 passing.
Having a young lady check you out in the coffee shop line and quickly return to her bored expression when she realizes that you're a couple age-groups above her is level 2 passing. Being addressed as "ladies" when when with GG's in a restaurant also rates a 2, in my book.
If you chat with a woman for a bit, and she says "Excuse me, you are a woman, right?", that's level 3 passing. Being referred to as "she" and "her" at the checkout stand or a sales situation is also level 3, in my book.
On a couple of occasions, I've chatted with women, and if they act shocked or surprised when you disclose that you're trans, that's level 4 passing, the black belt.
However, if you're in the ladies' room washing your hands and there's a long line behind you, and as you leave someone says "You know, I don't think that was a real girl..." You go straight back to level zero, baby!
(guilty of all of the above situations)
-- Diane