The real sign of being trans is if you go to egg_irl and start relating to almost everything lol /s kinda (sorry I haven’t got a clue how to link communities)
Tbh there’s no magic bullet to be sure, but if you fantasize about being a different gender that’s a pretty big one, cis people don’t do that.
I’ll add an edit: The only thing that confirmed it to me, was how excited I felt after I took hormones for the first time. Beginning them was terrifying, “what if I damage my body and I actually didn’t want this?” kinda stuff, but then I realised I wouldn’t have been waiting 6 months and been to all these appointments if I wasn’t, took the damn pill and felt amazing. It’s been ~9 months now and it’s made me insanely happy, that’s the only concrete proof I can offer to myself that I am in fact trans.