system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What are you seeing in a person's eyes that let's you know they are "staring into space" rather than into the distance or at a specific point?
gpt
We are _very good_ at reading whether we're being looked at - eye contact is a critical element of our communication (and very few animals use it - one of the craziest adaptations of dogs was learning to understand human eye contact).  The physical location of the eyes in relation to each other and you at short distances will be "off" to your perception if they aren't looking at you.  You can detect this instinctively.  

It's really two. things:

1. the rest of the face "snaps in" to communication mode when you're engaging with someone's eyes.  Eye contact comes inclusive of a general "we're communicating" that involves lots of facial muscles and actions.  The deadpan look of someone staring past you says "not engaging with you" because of this.

2. at short distances you can reverse engineer the way the eyes focus and know that it's not to you. This is why someone who is a little cross-eyed makes it really hard to know if they are looking at you.