Well they can be forms of multigender (multiple genders), or gendervast (monogender). Or it could be using genderqueer as an umbrella term and being a girl or boy. Genderqueer is for any nonnormative/noncis gender identity. So it could also mean demigirl or demiboy tied to ones queerness.
How you know? I can't tell! But it's part of self-questioning, self-awareness, etc. You can be both, either or neither. I'm both boy and girl, but sociopolitical reading, people group me with cis men because of my external appearance. Does that make me a cis guy‽ Well, biopoltically saying, I can't control always how others view/perceive me. Like in queercore movements, I'm situationist/condigender, depending on the context I say I'm cis or just a man, but in trans-inclusive and nby-inclusive spaces, I can say I'm both a man and a woman. And in some contexts, I'm fully abinary/neither, but the liaspec aspects are always there (such as in nixangi/nixavire), fragmented in small parts. And for some radinclus spaces, I can say I'm a transfeminine man, but this would lead to confusion for people with unfamiliarity of those terms.