There is nothing that is 100% proof.
Body disphoria can be a identifier, however not always. Some people do some strange thing to themselves thinking it will be better or that it is more "them" or may even cure them. So don't use that as an example of proof necessarily.
Liking to be treated as such is also not such a good way. There are people while to be treated as babies, they even wear diapers and learn to un-potty train themselves. Some like to be treated as slaves and used. Again, its a good indicator possibly but not conclusive.
Activities are also a bad indicator. My mother and grandmother like baseball, my mother played for a while. My dad couldn't stand most sports. My mom likes home improvement projects and has owned or asked for quite a few power tools.
Personally, I always knew I wasn't a boy, but I wasn't sure I was a girl. I couldn't make that leap. I did for a long time just want to be rid of "that" thinking if it were gone I could be happy. Eventually I realized it was more than that.
I have never liked being treated as a guy. I hated hearing sir, I hated hearing You're such a nice man, boy, etc.. It just never was me. I tried to act as one as much as I could, but it was always unnatural.
I was never into sports except bike riding and skateboards both of which have some girls in them. I also like older cars. Was I like the guys with some of this stuff, no. I wasn't competitive about it like they were.
Did all of that make me a girl, no. I had to figure that out on it's own. Only you can figure this out. You can do it alone, with the help of a friend, or a therapist. At some point you will just know I guess you could say.