Basically, someone who hasn't quite realized they're not entirely cis yet. "I thought everyone had dreams of being a boy/girl." "I don't get all that upset when I'm mistaken for a guy/girl on the phone." "I don't really feel comfortable hanging out with other guys/girls. It feels boring."

Stuff like that. Other trans people can (usually) spot an egg a mile away. But like everything else in queerness, it's never cut-and-dry, black-and-white, easily defined. They could always just be comfortable enough in their birth-assigned gender to not give a shit, and that's valid.

It's considered rude to assign this trait to someone you know personally, and the topic is only broached if the person in question chooses to be vulnerable and ask you specific questions about gender. Otherwise, they are (as always) your friend first, and treating them with care and respect always comes first.

The typical protocol when one of your friends says or does something a little 'eggy' is to quietly notice it, treat it neutrally and passively, and not say anything for or against it. Let them figure things out on their own. If they are trans, they know they can ask you things. If they aren't, then they're just happy and comfortable being themself around you. In either case, it's not a bad thing.