A few lessons I learned in hard way:
People who are very religious should not be trusted blindly. They actions are unpredictable and without logical explanation. They do not have personal integrity.
People who behave very politely should not be trusted blindly. They are often pretentious and has bad intentions
People who say mean things about others and use harsh language about their differences should not be trusted blindly. They will do the same to you and will take anything personally and will turn enemies quickly.
Give feedback to projects or works by others only if you know they will take it constructively. Giving honest opinions to some people will make you their enemy. Skip them, do not give feedback or optinion to them.
Observations about people's behaviors from my experience.
The sentences one person write or speak often gives clues about that person's intellectual depth. People who use very short sentence and often phrases instead of sentences are intellectually weak. They usually struggle to articulate relatively non-trivial ideas and events. These people are also often followers of hatred and extreme right wing ideas.
If somebody promise something to you and do it on time(or informs delay or not able to do it) are people with integrity. If a person does not value their promises, do not trust them with time sensitive delegations
People who have no opinion on anything are boring and not worth having friendship or relationships. People who have opinion on everything are pretentious and not worth having friendship or relationships.