You know you're trans if you just know that your gender is different than the one that was assigned to you at birth... If you're not sure, then you may be in questionning, and that's ok too.
Being trans isn't :
- "wanting to be of an other gender",
- "prefering one gender than the other",
- "not conforming to the stereotypes and gender roles associated with your assigned gender"
- "liking to dress ponctually with clothes culturally associated with a different gender than the one you were assigned at birth"
- link to sexual orientation or sexuality (= you don't transition because your partner ask you to, because it's arousing, because you (or your partner) don't accept your sexual orientation etc)
Being trans means you know somewhere deep inside that your assigned gender is not your real gender. Most of the time, body dysphoria tips you off about being trans, but not always : some trans people have very mild dysphoria, or no body dysphoria at all (but usually, you HAVE a kind of dysphoria - which can be social, linked to your name, your pronouns etc - or else there would be very low motivation to go through any kind of transition, to go through the oppression it means, the risk of loosing your friends and family, etc etc...)
Best way to be sure : think about it, about what you need, what is a problem for you, and what options you have to feel better...Think about the implications and consequences, about what it means... read testimonies and stories to see if you can or not relate with what people experience... And well, you can experiment too : trying to be called with an other name and/or to be gendered differently online : it costs you nothing, it's a very low-risk situation, and it can help you to see if that sound right to you... You can try wearing different clothes, changing your hair cut, trying your gender outside etc... And you'll see.. Just don't do anything drastic before being sure (like genital surgery ^^').. Even hormons can be tried and then stopped. Lots of people try and then stop, it's not a problem at all, as long as you're aware of the changes that can occure and be rather permanent (they're very few).
You can also see a therapist if you need help, or - better - trans listeners in trans organisations, they can help you too, sometimes waaay better than psychologists, since most therapists have no clue about transgender people...
No one can tell if you're trans, except yourself. And don't rush into identifying as trans (it can be quite harmful for the community when anyone tell too quickly they're trans and begin to speak for the community or try to raise awareness about something they don't know how to talk about...), it's ok to be in questionning. So respect that step of your work toward better self knowledge. Whether you're trans or not, at the end you'll know yourself better, and that's great :)