I see these questions sometimes, about "how can you be sure you're trans?" I see the sentiment often in articles that claim to "question" transsexuality, from well-meaning but pain-in-the-ass bystanders, and so on. A lot of it gives me a migraine the size of a T-Rex behind my right eye, so I thought I might help clear up some of the confusion.
There is only one way to "know" you're trans. And that's because you feel you are. Does this answer seem vague to you? It should, because it is. As much as others would like to tell you otherwise, there is no "test" for being trans. There is no criteria that if you check enough boxes, congratulations, you're transgendered. I ID very strongly as trans, but I fail nearly every single criteria required for Aetna's insurance coverage of trans care. In Aetna's eyes, I'm not a "true transsexual," but here's the thing: that makes me no less trans.
For some people, it is comforting to think that there's a trans test out there, that everyone who passes are trans, and the rest are just fakers, but unfortunately, humans don't work that way. If you fit into the simple boxes, congratulations, but if you don't, that's okay.
It can be frightening to think that the only way to know you're trans is to feel that way. After all, what if you're not sure? How can you be sure it's not just a phase, or something else going on with you? People change, after all. To that, all I can say is: know thyself. Figure out your reasons for why you feel trans. Does a certain word or pronoun feel "right"? Is it the way your body feels to you? Is it about gender roles?
It's okay not to know. Really. It is. Take as long as you need to figure things out. I encourage you to think about it, and think about yourself. I knew something was weird, gender-wise for... jeesh. Seven years? Something like that. It took me that long just to get my gender identity sorted out, and it was another one or two years after that before I felt comfortable identifying as trans. There is no hurry, no deadline, no day that will suddenly pass and too late! Your gender is now set forever! Take your time.
Also, your gender identity can change. That too is completely okay. Humans are versatile, adaptable little things, much more flexible than many people give them credit for. It doesn't make any identity of yours less real.
So, to make it all very clear now:
You can be queer and still be trans. Queer trans people exist; I'm one of them! So do straight trans people; I know some of them too. You can be gay, straight, bi, pan*, ace,** all of the above, or none of the above! Trans people come in all sorts of sexual orientations.
You can be gender-role nonconforming (a femme guy, or a butch girl) and still be trans. You can also be gender-role nonconforming and still be totally, utterly, completely cis.*** I know cis boys who like heels and skirts, and I know trans boys who like heels and skirts. I know cis girls and trans girls who're tomboys. Again, all of these things are totally okay.
You can feel completely okay with your body exactly the way it is now, and you can still be trans. There is no medical procedure checklist requirement. There are trans people who want every medical procedure under the sun, there are trans people who want maybe one thing and not another, and there are trans people who want nothing to do with doctors. It might be a money thing, a health condition thing, or just plain "no thank you" thing. All of them are equally okay.
As far as body shapes go, trans people come in all shapes and sizes. There are trans people with square jaws and big muscles, and trans people with curvy hips and full lips--and some have all! There are also cis people with these things. None of it should be an indication of "how trans" or "how cis" you are. If people call you the wrong thing on the street, well, that sucks, but that doesn't make it true. If people call you the correct thing on the street, that's great! Congratulations! But that doesn't make you better or "more trans" than anyone else.
You can be a mother without being a woman. You can be a father without being a man. Wanting to be a parent and be called a certain thing doesn't mean your gender identity is a lie. It just means that you want to raise your child or have them in a certain way. And if you're a trans man who wants to be a father, or a trans woman who wants to be a mother, or a trans person who wants to be a parent, you can be that too! All combos are totally okay!
Trans people and cis people come in all sorts of amazing, diverse forms, and all of them are okay.
You might be trans, or you might not. You may know immediately, need some time, or maybe never know for sure. And you know what? All of those things are okay. Just do the best that you can, and try to get to know yourself as best you can, and you'll probably be just fine.
*pan = pansexual
**ace = asexual
***'cis' means 'not transgendered'