Well, the only thing I pass is gas, so I wouldn't be offended if someone read me. Usually my reaction is either asking if they figured that out on their own, or acting surprised, reaching for my crotch and saying "Wha... Wa... Wha... Where the hell did THAT come from?!?!". What does offend me more is when I'm treated as something other than a trans woman, namely a gay man or a man dressed as a woman. What I've been doing when I get misgendered and I get a certain type of "vibe" from the person is act surprised that they're gendering me as male and stand on my ground of being a woman, with no mention to my trans status. It works surprisingly well.
But I'm drifting away from the main topic. I know not everybody has my thick skin, so I wouldn't approach anyone just because they seemed trans, specially if they passed "quite well". And then, there's the many many ciswomen who have all those tells that supposedly mean you're trans. I know a bunch of those. One of them is my cousin. She gets read as trans pretty often, despite having popped out two little humans already.
To be honest, the whole obsession with passing (and that transpassing subreddit) is, for me, the worst we trans people (specially trans women) can do to ourselves.