Several suggestions here are pretty good I think. I wouldn't recommend the eight ball for anything important and I wouldn't recommend depending solely on my gut for making a major life decision like marriage.
Research is good if something is researchable. Research what you should know about something and then research the components until you know what you should. After that, the decision will usually be obvious.
Before forming a partnership of any kind ask questions to get answers about anything you don't know. If it turns out the National Enquiror is your prospective new business partner's primary news source and he consults fresh chicken innards before making a stock purchase, you can be pretty secure in whether this is a match.
And for some things I just lay a fleece the way Gideon did in the Bible.