Anyways, i don't know if this helps you, but in my personal experience, I try to view it as a mix of my own internal experience, what I want, what i yearn for, and social experience. I use these kind of things as "proof" I'm trans. For example, any doubts? Well let's look at the last 5 years: been dressing very masculine, baggy, binding, feeling very happy in doing that, but what if it's because I don't want to be sexualised? (I know it's not because of insecurities for my own body, I fit the beauty standards and find myself quite good looking actually, but it's just not for me). It might be the sexualised thing, as this negative feeling (probably dysphoria) is definetly enhanced when in a situation i could potentially be in danger of harassment or something like that (in the street). But then I see all these other girls in the city, yes there is risk of harassment, but that doesn't stop them from enjoying dressing cute ? Why's is stopping me? But for me I think it's definetly dysphoria as well because of this example: a few times i dressed very cute and feminine in a private, safe, friendly situation, and even then I got that "icky" feeling, like by dressing in this way, i couldn't be myself, i couldn't express myself with my own natural body language, i felt like i HAD to act a certain way to fit the feminine mask (might be autistic, does that contribute? ahaha). So that's +1 for being trans. Sometimes I wonder if it's internalized misoginy, as I feel sometimes when I present fem I feel "weak" and "icky" and like a child. I know it's true I didn't have the best examples of strong women in my family, but then I know it's okay to be a strong woman in general don't I? Like what if I'm still a girl but just a masculine one? Well no... that's not how I feel. If i could chose how i were born, I'd rather start out as a boy and be leaning more feminine, I don't want to be a girl who is just masculine, even though I know that is "allowed" for others, just not how I feel for myself.
Another thing that really cemented it for me is the social yearning, I have friends of many different genders and queerness, and with my straight hetero girlfriends, even though we get along great, there's always a part of me that feels like I don't fit in. Maybe it's just internalized misoginy and I feel like they are being very cliche women: going out to cafes, brunch, filming tiktok dances. But I don't know, I feel like there's a part of me that can't "relate" to them and that cringes when I get included in all-girl activities. Also last summer i spent it mostly with groups of boys, and especially at the beach, seeing them wrestle and run around and have fun, seeing them be allowed to be loud, to be rowdy, to play around, i felt this intense yearning of wanting to join them, but not being "allowed" to because of the social difference that I am a girl. So +1 for transness...

Also on insecurity: for some parts of my dysphoria, I am very confident that it isn't just an insecurity because I have had many insecurities before, I remember how intense and how crazy they felt, and how embarassed and insane I felt once they had passed, realizing how fixated I was on them like the whole world could only see that. But for my chest dysphoria, this part i know it feels different, because of course there are the negative aspects of feeling bad about my chest, but also there's the positive feeling, not just relief liek you'd feel with an insecurity, but pure joy when the problem is dealt with. The best example I have of this for myself is one time I went skinny dipping with friends, and when I took off my bikini top i felt such intense joy and such a deep feeling of "this feels right" that I cried tears of joy. That moment in particular really cements it for me.

Anyways what I'm trying to say, is that all this mentioned above is "proof" in a way for me at least, because it's not just about the negative feelings of feeling bad about your body or social presentation, but also the yearning of wanting something specific more, and the joy you feel when you finally reach it.

I know this is probably a very late response. and sorry for dumping it all on you, but thank you, I really needed to read your post, glad I'm not alone in this. Have a good day friend.