This is basic google stuff. What is trans?
Advocates will say being trans is "people whose gender identity is different from the gender they were thought to be at birth."
How do you know you are trans/have a different gender identity? From transequality.org (this is the ENTIRE answer):
People can realize that they're transgender at any age. Some people can trace their awareness back to their earlier memories – they just knew. Others may need more time to realize that they are transgender. Some people may spend years feeling like they don't fit in without really understanding why, or may try to avoid thinking or talking about their gender out of fear, shame, or confusion. Trying to repress or change one’s gender identity doesn’t work; in fact, it can be very painful and damaging to one’s emotional and mental health. As transgender people become more visible in the media and in community life across the country, more transgender people are able to name and understand their own experiences and may feel safer and more comfortable sharing it with others.
For many transgender people, recognizing who they are and deciding to start gender transition can take a lot of reflection. Transgender people risk social stigma, discrimination, and harassment when they tell other people who they really are. Parents, friends, coworkers, classmates, and neighbors may be accepting—but they also might not be, and many transgender people fear that they will not be accepted by their loved ones and others in their life. Despite those risks, being open about one’s gender identity, and living a life that feels truly authentic, can be a life-affirming and even life-saving decision.
"They just knew."
It's all about "when you realize it," or "how you know." There is not a single concrete, testable, or provable thing. If you feel like you are a different gender, if you feel shame, confusion, fear, etc, then you are.
The description of WHAT IT IS TO BE TRANS is a description of gender dysphoria. "They just know" that they don't match. They can tell. They sense it. They FEEL the discrepancy.
That's unease. That's distress. That's what these words mean.
I don't care that the clinical definition of gender dysphoria calls for higher amounts of suffering or distress. They are talking about the exact same things.
Gender dysphoria is trans. Trans is gender dysphoria.
And gender dysphoria is a mental disorder.