What, to you, is the 'tell' that makes you think "Yes, this person understands how to make good characters?"
For me, it's detail with no context. Character reactions to things, tell me a lot about their personality and history. If you have a good backstory, you're set in my eyes.
if your backstory explains, in depth, your character's personality, you're gold. (More than just "she went through x trauma, so she feels and reacts like x." I'm talking like what little things happened that effect their method of speech, what went through their heads and changed when the small stuff happened? Those are good details)