It really bothers me when people object to the idea that people “just know” their gender. A lot of cis people try to turn gender into something diagnosable, getting annoyed at the idea that gender is just something you know, that a man is someone who knows himself to be a man.
It bothers me because I almost never see this argument for sexuality, or other similar things. Most people are willing to accept that a woman knows she’s a lesbian because she’s attracted to women, which is basically saying that she knows she’s attracted to women BECAUSE she’s attracted to women.
I realize that there’s the argument of physical response- but that’s not all there is to attraction. It’s possible to think “I would like to have sex with this person” without immediately getting physically aroused, or to be around someone you are attracted to without constantly being horny. Physical arousal is part of it, but it’s more than just that. You know you’re attracted to someone because you’re attracted to them, it’s tautological but true. It’s the same with other parts of identity.
There are indicators. Dysphoria or a lack thereof, what causes the dysphoria, what alleviates the dysphoria, etc. And, yes, it is based on social ideas of what a man and a woman is- however, it can be based on very broad ideas of what each gender is. Women who wear pants and play football and cut their hair short and do stereotypically male things are still women, men who wear skirts and bake and wear their hair long and do stereotypically female things are still men.
I see a lot of cis people who make this complaint seem to expect that the answer is “transgender men are just women who want to be more masculine, transgender women are just men who want to be more feminine”- which just isn’t true. Gender isn’t something that can be wrapped in a nice little bow because of the way we see it, it’s complicated primarily because we’ve set such narrow constraints on it and now that people are pushing against those constraints, our models and language for gender are just not enough.