While I don’t want to focus on getting ‘clocked’ as a defining part of the trans experience, it is an event that’s almost uniquely trans. It’s a source of uncomfortable dysphoria, and is one way you can add relatable pathos to a trans character. When I wrote a trans characters, I found a few sexually dimorphic (but not sexual) characteristics of that character’s species to give hints to other characters as to their identity that someone living in that world would be familiar with. Friends might avoid drawing attention to it, while unkind or unaware outsiders might mention it.
One other thing that’s important to express is that there’s an opposite to gender dysphoria, which many trans people I know label ‘gender bliss’. This is the joy they feel when they are able to see themselves in the right body in some tangible way. Sometimes it’s something really small, like getting their hair or nails right. Or getting their voice right, or passing to someone they don’t know. It’s easy to write a character that’s all pathos and drama, but they’re incomplete without the other side of that coin, too.
Please recognize that I’m talking about these experiences in a very utilitarian fashion. Both passing and getting clocked can be a very personal experience, and you’ll want to use it sparingly. Have it happen once. Just like other character traits, make sure that you don’t treat being trans as their -one and only- character trait. You’ll end up with a stronger story if it’s something that comes up once, creates the appropriate drama, is dealt with, and then returns to set dressing while the character’s other interests take focus. It can be really simple and low-key, and it doesn’t even need to involve the character directly.
Unaware character: “Hey, is that -deadname-? He’s looking, uh, different.”
Friend of character: “That’s -charactername-. She’s changed a bit since you knew her in high school.”
Previously unaware character: “Oh, sorry, I didn’t know. Um, yeah, okay. She’s really rocking that dress tonight.”
Perhaps your best tool when working with a trans character is subtlety, subtext, and implication.