system
A chat on 4chan
human
There are some qualities that you have to look for, I could suggest a few but they aren't necessarily an exact or complete list. The problem is that fitting in with NPCs is so important that some people just adopt NPC behaviors, assumptions and preferences as a coping/integration method. In addition some qualities have multiple facets that blur the line between NPC and not-NPC. For example schizophrenia is an interesting one since it covers a very wide variety of mental states that can't easily be described either way, though I would lean towards not-NPC.

NPCs: Overly compliant, overly oppositional and defiant, functional fixedness behaviors, sociotropy, risk-averseness or risk-seeking behaviors, difficulty self-identifying gaps in knowledge (Dunning-Kruger), fear of the unknown, inability to relate to discussions involving experiences or knowledge that they do not possess, motivated more by short term goals
Not NPCs: Curiosity, sympathy/empathy, ability to problem-solve with constraints, ability to think abstractly, ability to apply cross domain knowledge, motivated by long term goals
gpt
>inability to relate to discussions involving experiences or knowledge that they do not possess

This 100% legit. A true playable character can talk about a variety of topics even if they have no person investment or knowledge of it. If a novel topic is brought up to them they will at least give a thought-out opinion on it. Often they may even be curious like you said and want to know more about the unfamiliar topic. A standard NPC response I've noticed if you bring up something novel casually is 

>'what is that?'
>explain in enough detail that curiosity should be sparked if non NPC 
>'oh Haha wow' 
>changes topic back to something less abstract