You are the expert on your gender
The road to knowing one's gender can be long for some, short for others. Some people are born, assigned a gender at birth, and are pretty happy with it. They might never wonder if their gender really matches up with who they are. Some might question if being assigned female, for example, is really accurate, then realize yes its congruent with their personal identity. For others, gender discovery takes a lot of time, work, and sometimes challenging others' opinions about their gender. One thing is certain though: only the individual is the expert on their own gender. No one can look as someone and know what a person's gender is.
Gender can be fluid or fixed. A person's gender can change over their life, or even day to day. These are some terms or labels people choose for themselves:
Cis gender: gender assigned at birth aligns with person's gender identity
Gender fluid: a person's gender or gender changes
Gender queer: umbrella term for someone who does not align with conventional standards of gender, can be no-binary for some and not for others
Non-binary: gender that is not exclusively boy/man or girl/woman
Transgender: anyone whose gender identity differs from their assigned birth sex. It can also used more narrowly as a gender identity that reflects a binary gender identity that is “opposite” or “across from” the sex they were assigned at birth, though some trans folk who identify as non-binary use this term as well.
Language is always evolving, some of you might remember when people felt it was correct to use the term trans* with an asterisk, which as since fell out of popularity. The word queer also has some negative connotations for some people, as it was a slur used in the past which has since been embraced by the community as a way to strip its power.
Again, the correct way to know someone's gender is to ask them, not make assumptions. People get to choose the labels that fits them, not what other people think fits them best. Getting into the practice of offering up your own pronouns when you meet someone can be a friendly way to communicate that you realized your gender isn't assumed, and you don't want to assume the other person's, either. Also adding your pronouns to name tags at things such as conventions can help normalize the habit for other people. Some people may not feel comfortable in sharing all their labels, which is totally okay.