There’s no one easy way that works for everyone to be sure. You have to find your own answer. Being trans isn’t an externally measurable truth, it’s something only you can ever know. I knew after I transitioned and then briefly detransitioned. The 2 months I spent off hormones was almost 6 years ago. I’ve never once regretted retransitioning. I didn’t know for sure, I took 2 months off hormones and using my deadname, and it was never a question for me again. The pain I felt every time I heard that name told me everything I needed to know. But for me thats what it took to be sure. Self doubt lingered for me even after starting HRT and changing my name and pronouns.

But thats only me, and everyone has their own unique experiences of self-acceptance. You have to introspect. Open yourself to the possibility that you’re not cisgender, and ask yourself what it is you really want to be, who you really want to be to yourself and to everyone else. Try using different pronouns. Try going by a different name. Dress differently, do things you want to do but have always felt like you couldn’t. Play with your expression and presentation. Do things that make you feel great, and things that make you feel great about yourself.