I've said this elsewhere, and it sounds arrogant, but it's actually common business sense:
I want to directly contact the principals involved and have an in-depth conversation with them.
If I'm going to put my name on the line on their behalf -- investing time and money in their enterprise -- then I want to know with whom I'm dealing. The good ones have no problem with this. Among other things, they know I'm serious about working with them.
This is a hard and fast rule for me. Under no circumstances do I want to recommend a product or service to anyone that I haven't researched thoroughly, use myself, and can explain every aspect of it from direct experience.