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.