You find out in baby steps. You trust them with something small....then move up. That's pretty darn normal in my opinion......it's how I do it anyway.

As for how can you know? You don't get to. That's what it is ALLLLLL about. That's why relationships are risky. And take alot of work.

Sorry unknown, no easy answers and only one piece of advice. Study the hell out of your mistakes so you can learn from them. It's the one thing that will keep you from repeating the same error over and over.