Reading people is a survival skill
The hard way.
These are the things I wish I'd known going in.
1. If a person isn't nice to the waiter, they aren't nice.
2. If you learn a person has lied to you, confront them. If they lie to your face when confronted, then they have revealed their character. File that information away, and never, ever trust their words.
3. Insecure people are dangerous people. They will throw you under a bus. Everything threatens them, so for them it is simply a survival tactic.
4. If your boss asks you to do something you aren't sure is ethical, don't fret or feel angst about it. Just say "No. I'm not comfortable with that." Then, find a new boss.
5. If someone on your team presents your ideas as their own, do not fight for credit. (People always assign credit to the first person they hear the idea from, even if they find out better later.) You have just learned that that person cannot generate their own ideas. Then make sure that you only share your ideas in ways and in places where there are others to verify credit. (Also make sure you are always generous in assigning credit to other people for their ideas.)
Note that blogs are good ways to document ideas. They are public and they have publish dates.
6. If you stay too long in a bad ecosystem, it changes you—and not for the better. It affects your health and your personhood. No one is ever truly stuck. You may have to radically rethink your lifestyle, change geography or do something you think is beneath your education level or experience, but it is possible to change. (And it is worth taking those steps.)
7. A good ecosystem is worth staying in—even when offered more money elsewhere.