Which Basket Should A Developer Put Their Eggs In?

The situation goes like this. Lots of excellent, capable people have great ideas for new software products. They are bootstrapping their venture, so they look for a developer who wants to give some time up front and receive a fair (let's assume fair) share of the rewards once the product becomes successful.

The question is: as a developer, how do you know which of these products (I get about one tempting enquiry a month from what sounds like a real person who isn't building a social network) is a good bet?