I think it's easiest to "judge" them in the context of your personal goals. So, rather than thinking about if they know what they're doing, look for indicators

I think it's easiest to "judge" them in the context of your personal goals. So, rather than thinking about if they know what they're doing, look for indicators that they'll provide an environment suitable to whatever it is that you're there for. If you're there primarily to learn, for example, then different characteristics will be important than if you were there to get rich. All that being said: Yes, the fact that they've done this before is a good thing. It doesn't really matter if they failed or were successful in their last venture; both types of experiences are educational and should help them in this new project.