For me, I don't have control over the gender my hindbrain bins people as. I can't consciously decide to view people as the gender they want me to, if it's different from how they look. The people that successfully confuse my brain into 'no gender' are mostly mid-passing trans.
Right, but I mean did you read what I said above? If someone looks like a woman, and then says 'I'm nonbinary,' this does *not change how my brain treats her*, and I don't consider it a meaningful statement because it doesn't correspond to anything in the real world.
I can't give you a 100% answer; sometimes I can't tell if someone looks like a woman or not. Usually it's in the facial features - narrower jaw. Often it's in the build - breasts, larger hips, etc. I can guess with pretty high accuracy what chromosomes the person has.