Ok so human-form titan!Eren keeps several of his titan-form features like his hair, eyes, and ears, and of those, only his ears give him away his true nature.
When he was a little child those ears were disproportionately big for his size, but eventually he grows into them and they stop looking quite so silly/cute and start looking more regal.
Which is a running theme as Eren matures over the course of several years right in front of Levi’s eyes. It takes him a long time to notice, because his platonic relationship as Eren’s guardian/mentor is so deeply engrained. The little feral child he took in flourishes into an attractive adolescent and then into a very handsome young adult. Everyone else notices this long before Levi does, though what doesn’t escape Levi’s attention is how Eren is quickly becoming a distraction to the young recruits whenever Levi brings him to the field.
It’s annoying as fuck, because those recruits should be focused on Levi and what he’s saying, not gawking away at Eren. Why yes, Levi supposes they are probably fascinated by the famous human-shifting Rogue Titan, but…what’s that, Petra? What do you mean they’re not staring at Eren because he’s a titan?
…oh.
Oh.
When Levi does finally notice how well Eren has grown up, it hits him like a freight train.