I'm a young guy and not very social nor do I say I have good social skills. However, if there's one thing I have learned is that you can always know if someone's a good person based on how they treat animals, neurodivergent people, and people who hold less power than them.
I'm not talking about very obvious stuff like "Guy" goes kicking around puppies or throwing around the R slur all willy nilly. In my experience and especially in high school people will subtly expose themselves, but I don't think people usually notice.
Example 1, killing small frogs or even lizards or animals that aren't stereotypically cute. Most people don't really care if you kill bugs like cockroaches or flies, which are normal since they are pests. However, taking pleasure in killing small amphibians when they aren't a threat for pleasure is not right.
Example 2, liking and finding humor in posts or jokes that stereotype, fetishize, or joke around neurodivergency. It's usually someone jokingly saying someone is actually autistic. Furthermore, they treat neurodivergent individuals like animals and try to stress them out by bothering them for their sick amusement
Example 3, the way people treat others when someone is alone, minding their own business, or just introverts in general is incredibly telling. I'll use an example of when I was studying on a bench alone during lunch break in high school and a gang of guys went up to me and mocked me about studying. This tells that these people will act rudely when in groups or wield power like physical strength over another party.
I will also emphasize on this as 1-person vs a group of bully will hurt more, even if the 1 person doesn't know anyone in the group or cares beyond the group bullying them. Humans are social animals and being ostracized even from a group of people you were neutral to at most is never a good feeling.
People will often go with the flow or go be assholes with whatever they believe is still in their boundaries of still being a moral and good person without crossing in the line of questioning. Furthermore, people who lack critical thinking skills and sometimes people who are religious (NOT ALL RELIGIOUS PEOPLE THIS IS AN EXAMPLE) will twist meanings, beliefs, or even tenants/rules to suit themselves to being assholes without worry or questioning if they are a disgusting person. This is called cognitive dissonance where people experience discomfort from holding two contradicting beliefs.