Honestly there’s not really a way to know short of them telling you. There’s a difference from folk just not liking the gender box people put them in and rejecting all the cultural trappings of gender (being a tomboy or a femboy) from them being trans. Transness goes a little further than just cultural markers, it’s a reaction to one’s body. Oftentimes that struggle on the outside just shows up as them not flourishing… And sometimes you don’t recognize what them actually flourishing actually looks like because they never did until after they changed.

I grew up in the 90’s and from sheer lack of exposure just didn’t have words for what I was going through. I was aided by being fairly androgynous but really didn’t talk to anyone about how good it felt to be read by strangers on occasion as my gender. I relied on gender neutral nicknames. I starved myself or overexercised to stay lean at points to keep myself from putting on weight that would go to areas that would outwardly show my body through clothes and avoided mirrors while naked but none of that clicked as me being trans until when I was 21 and living abroad in Japan where basically everybody read me as being what I was, either assuming me as a trans man or reading me as a cis man. None of this really caused me to self reflect until I was near the end of my visa and realized that going back to all my friends and family whom I loved dearly was a double edged sword. I would be locked back in to where people would enforcing my gender, lightly mind you. They weren’t trying to force me to act any way at all but there was a gentle tyranny just by them correcting people who “got it wrong” or using my name or by men I saw as friends and peers treating me as a delightful oddity like I was some sort of ideal romantic though not nessisarily sexual conquest because I liked hobbies and masculine dominated spaces that few women participated in which in modern context would probably outwardly make me appear as some kind of “pick me”. This realization that I didn’t want to go back cascaded into me crashing hard up against all the novel fantasies I had neen distracting myself with that I would somehow go through some kind of magical event and instantly change body type and all my friends would just have to except me because “oh well magic…” I never believed this would actually happen mind, I wasn’t delusional but I would amuse myself while walking around with these little daydreams. All at once though I realized that that was never going to happen. I was gunna be in this form until I died and I broke into a full on dispair. I didn’t even know trans men existed and my only experience with trans women was representation where they were ridiculed. I backwards engineered that trans men must exist because that was the only thing that made any sense.

I stuffed it all under my hat for another 10 years, growing more distant with old friends and not making new friends. I read a bunch of feminism and chased out my internalized misogyny thinking that was the problem. It muddied the waters awhile but I couldn’t shake that no matter how I told myself that being a woman and being a man were value neutral it didn’t shake my feelings like I was playing out Franz Kafka’s Metamorphosis and all people saw was the roach. I tried non-binary pronouns and a name change more or less as proof to myself that I was okay without and discovered the opposite.

My mom took me coming out hard only in the way that she felt she should of seen it sooner and it threw into sharp relief all those times where she’d tried to pressure me in little ways to be more fem. I don’t begrudge her any of that. She says it should have been obvious but really no. If I had known that there were options I could have asked instead of hurting myself the way I did and struggling with the isolation then I might have. But I lived in a conservative town where just growing up in an agnostic household had seen me get literally have neighbor kids throw rocks at me growing up. Even if knew my friends and family were cool, there were medical options that would reduce all the regrets that I have now I might have buried and denied my needs anyway. My family had kept me alive by being awesome in other ways and I always knew that me dying would have destroyed them… And that’s really all you can do. Let your kid know they are loved regardless of anything and let them sort themselves out. No need to brace and seek the signs one of them potentially trans, just let them know that you love them and if they are then you will still love them and want to do right by them.

I have a trans son, and really I don’t think you need to worry about it - if you are caring enough to even ask this question, your kids will be fine, just let them grow up, they will figure themselves out. I literally just thought my youngest was a butch lesbian not a boy, my concept of womanhood is broad, he had to “come out” and he’s not at all traumatized by that.