How do I know my gender dysphoria is real and not something else?

Self-awareness is very important in this. A first step is to identify where this insecurity comes from, what else you think your dysphoria could be. And then, you can get support from people who know these matters very well:a therapist would be a great option, particularly one specialized in LGBT topics. The trans community can also help you figuring things out by sharing their experiences and comparing them to your own.

Usually, if it continues past the age of 13 or so, or it might be past adolescence, you will know by then that it's real. It can be very difficult to process and a lot of people end up not feeling like they have gender dysphoria, but it really takes a lot of time to discover who you are and what you define yourself as in regards to gender. It's best not to hyperfocus on it. You can also ask people that are transgender and adults or people that have possibly experienced gender dysphoria. I hope this answer provided some assistance.