It's human nature to judge others and while we may not always know what we're judging others for, research shows that it's usually small, subtle things they do. Some things we judge other on are:
How they treat waitstaff and receptionist - For example, if you are interviewing a potential employee for a job position, they are most likely going to treat you with respect. However, how they treat the receptionist on the way in and out of the building shows more of their character.
How often they check their phone - If you're having a conversation with someone and they keep checking their phone you judge them as being disrespectful and having a lack of will power.
Their handshake - You may not realize it but if someone's handshake is weak you probably associate them with having low self confidence. A stronger handshake usually belongs to someone who is more extroverted and less shy.
If they're late - When someone shows up late you tend to categorize them as disrespectful, lazy, and disinterested.
How much eye contact they make - If someone does not make enough eye contact with you, you may perceive them as disinterested or embarrassed. On the other hand, if someone makes too much eye contact, they can come across as creepy.