The main thing I look for is the same thing I do when I'm either side of a job interview. I do what's called a "two-minute psychometry". It simply consists of communicating with the person while taking on various personality traits. When you get strong, fast agreement, that's the person's base-personality. Run like heck from the destructive ones, pick the positive ones. Hasn't failed me yet.

Takes a few hours to learn properly, and some practice to get good at, but it's easy, it's accurate enough, and it works.

Even a "failing" company, with the right leadership, can pull out of disasters. Can be worth the chance to join a company that very well might showing signs of stress, if the leadership has the traits that can deal well with that kind of thing. Even a prosperous company that's currently doing well can be killed horribly by leadership with the wrong traits. So I pay more attention to who I'm dealing with than any other single factor.