You know you pass when you're going to get your name and gender marker updated with the Social Security administration, and when you present the clerk with your therapist's letter to change your gender on file, as well as the court order for your name change, he smiles and says "I never would have known." (plus uses correct name and pronouns the whole time, and speaks to you in that endearing "talking to a pretty girl" tone that guys use, despite knowing that you're trans and even knowing your old name.)
That made me feel pretty darn good! π