How should I show that a character's nonbinary when they themselves don't know? Should I even do it if they'll just be misgendered the whole story?
They're born in the early 70s (when the main story takes place they're 22). They have trans relatives but I don't think they know there are options beyond (a) staying their AGAB, (b) transitioning to the other binary gender, or (c) drag, which isn't really their thing.
I want to make it deeper than "they think gender roles are stupid" but I don't know where to start in portraying an nb egg.
[I'm assuming a lot about the culture of the particular 70s you're referring to. My POV is that I am "canadian" and much of my own personal knowledge of transness in this era comes from research in US archives. YMMV when it comes to places outside the heavily American sphere of cultural influence.]
So first of all, the 70s had a bit of a renaissance of androgyny. A common thing I'll see in advice columns of the era is someone asking what to do as a customer service worker when they "can't tell" what gender they think someone is, because of their hairstyle and the way they are dressed. I imagine there probably were a lot of people who took refuge in that ambiguity. A fro, a pageboy haircut, all these are something that people of any gender (not necessarily every texture but I digress) could wear.
Secondly, I figured out I was nonbinary before I knew it existed, before I had the words for it. (Though the term genderqueer was the in vogue one in the community at the time I figured my stuff out.)
It's just that I sort of saw it as, "hmmm, well, I feel kind of between these things." And I think I'd seen it as sort of "half-trans" until I really understood transness better. (Note: this was for like a week tops.) I also went through a period of time a few years before this where I just sort of saw myself as a crossdresser (not drag - just someone who chooses to dress and cut their hair as they are comfortable and feel happy), but like, that as my gender identity, if that makes sense. I didn't have access to community of other people who felt like I did in order to compare to. But at the time, this also meant that when I talked about my feelings with others, they wouldn't have the same hangups about what it would mean for them if they said they felt the same. There were a lot of people I talked to in 2011ish who basically said they experienced gender as I did, only, I don't think most of them would understand for another 4 years at least.
One day I'd like to post more info about nonbinary history (especially in the 70s), but I'm just dumping info at this point. Something I will suggest for more on the topic is reading Lou Sullivan's diary, and reading autobiographies from trans people who were alive then. Even when they weren't out, they still existed, and lived in a world where they were carving out spaces for themselves.
To my understanding also, a lot of trans culture at the time sort of distinguished "transvestite" (trans people generally? but who haven't accessed medical transition) and "transsexual" (trans people who have or are intending to transition medically). Trans people of every variety might change gender expression based on their outness or the safety they had, and this didn't make them less seen as trans per se. A lot of people who ID'd either way (though more for "transvestite") would have a variety of approaches to things like name and pronouns. If this were the dominant terminology of the time that I were figuring things out, I'd probably have called myself a transvestite, though I would have likely been questioning the transsexual label for a while and ultimately not been able to access medical transition. (Though it depends on when in the 70s iirc - there was a time earlier on where it was easier to access I believe, but I'd need to fact check.)
In terms of general nonbinary egg mode stuff, or at least egg for an era where being nonbinary is not widely known to be a thing, here's some 'signs' (some are just straight up "that's canon if you put that in") I've brainstormed.
admiring specific fashion trends and looks that are gender nonconforming or androgynous (especially celebrities - maybe glam rock musicians, Joan Jett) (Joan Jett was huge for egg me personally...)
finding a lot of different ways to express how they feel in words (the "man/woman in a woman/man's body" phrasing is something that has historically bugged me, but people have used it throughout history) (more examples, dependant on the person, "I feel just as un/comfortable in my body and the way people see me as I would if I'd been born differently." "It's not a wo/man's haircut, it's mine." "I feel like a guy among my guy friends and a girl among my girl friends." "I feel uncomfortable when I go out with my partner and we are assumed to be straight/gay, though I don't know why." "I really enjoyed breeches roles when I did theatre in high school." "It felt good when I was mistaken for a woman.")
referring to themselves in their head with neutral terms
having dreams where they are recognized as themselves
feeling at home around trans people and queer people in general
watching or listening to certain songs/movies/etc that feel Gender for them over and over again
some kind of fixation on facial hair/other body part or lack thereof
it feeling different when different people use gendered terminology for the character (ex: a conservative Girl Guide leader calling everyone "ladies" vs a gay man saying "hey girl"). this is usually to do with what it's assumed the speaker's assigned meaning to the word is.
Re: misgendering through the whole story. A really convenient way to curb this would be to just have the character feel like the way they are (mis)gendered is also a part of their identity, it just isn't the whole picture. Another thing I sort of thought of in my early years was like, if people think I'm a woman, at least they understand there is something off about it. Whether they think I'm a butch lesbian or intersex (which was very regularly assumed throughout my life, at least until the general public started to catch wind of genderqueer identities). It matters to me that they at least understand my approach to gender is queer.
Another approach would be to have the story written from a distant past tense where the reader knows the identity of the character, because everything is told in distant hindsight. This is my favourite approach personally.