Estella is kinda shallow, so usually its something regarding their fashion. If they have a recognizable style, if they utilize jewelry, etc.

Aside from that it'd be something regarding competition. If they are a trainer it'd be how many balls are on their belt if any. If they look like they're experienced. etc

How trusting they are of strangers.

Elio has years of being a socialite and a conversationalist under his belt; it stems from his siblings' perfectionist mentality. So, at a glance, he would notice various details about a person, which range extensively - depending on the individual. Typically, this means that he's a person that can "read the room", and act accordingly.

He does his best to be as subtle as possible when mentally picking up on details about people he just met as to not make them intentionally uncomfortable.

Their eyes! Yorra has a very bad habit of staring because of it lmao, girl will make direct eye contact and won’t break it unless you call it out.

For Jin, the first thing she notices when meeting new people is their aura. Which is also one of the reasons she's usually such a good judge of character.

Balei is all about personality, typically she enjoys the outgoing types

Meanwhile Fausto is comparing heights…

How they present themselves!!

Nowadays, since most new people Odd meets tend to be customers, he pays attention to their order and how they intact with their shop

Odd stands by that you can learn a lot about someone through their preferred drink (what it is, why they order it, is it for here or to go, are they particular to how it's prepared, etc.), and since more often than not the shop displays various curiosities, more often than not there's one that catches a customer's eye (which helps Odd narrow down what kind of things spark their interest)