There’s not any sort of checklist to go through to see if you match up with enough things to be agender, and no one else can tell you your gender and who you are, it’s something you have to find. There is also a variety of ways people experience their gender.
For me specifically, I really disliked the way people talked about “coming into” my assigned gender around puberty, and I disliked being identified as my assigned gender. It felt kind of gross and uncomfortable sometimes. It wasn’t that I couldn’t be that gender, it just didn’t feel like it was actually me. I never understood what people meant about “feeling” like a man or a woman, or what gender really meant. I thought that because I felt like I could live as my assigned gender and I wasn’t bothered much by my body (in relation to gender, I disliked it for other reasons unrelated to gender) that I couldn’t be trans, but I read more things about the issues with defining “trans-ness” by discomfort and misery and how suggesting that someone could only ever be trans if the alternative was unlivable is harmful.
Even if it’s more difficult sometimes not identifying with my assigned gender, and even if maybe I “could live” with it, I feel much more right saying that I am agender. I feel like I’m representing myself better, I like not being identified with my assigned gender, I feel more like this is me than before.