Honestly, just tell them to let you know, in private if they feel it necessary, if you make them uncomfortable. In general, jokes about people being trans/gendernonconforming are ok, as long as it’s not the one “joke” – identifying as an attack helicopter/dragon/ridiculous things. Pretty much just don’t make fun of them for being trans, but it’s fine finding humor about their transition, if it makes sense. It’s generally pretty easy to tell when someone has a problem with you being trans and is going to be a dick to you under the veil of humor, and when someone is joking around with you. Just make sure they know to let you know if you make them uncomfortable, and tey not to be a dick. The fact that you’re asking makes me think pretty much anything you’d consider saying is forgivable at worst.