Perhaps past employers might work, but I think what you say about their track record is really the only way, outside of just knowing someone or a referral from someone you trust.
Listen carefully and dig deep. Don't just settle for "I know this guy and he made me a bunch of money." Ask the reference what the guy did to make money, what was his plan, how they met, why he decided to work with him, why they aren't working together any more.
Imagine you're sitting down for lunch with the reference and having a conversation about the person he's referring and you'll do a better job of understanding what he's all about.
Interviews are also notoriously bad for hiring. They may make you feel better about a person, but they arent actually better at predicting future outcomes most of the time.
History and Examples are much, much more valid for everyone. For example. who needs references when I could just SHOW you something I did. What would you trust more -- some stranger, or your own eyes and ears?
I've had similar concerns and got lucky meeting people through either acquaintances or through work. So before there was ever a business partnership relationship I got to see how the person worked with customers and other coworkers.
Good luck.