Surely a lot of bookers DON'T go on what the act sounds like but on their reputation.

We had a posting here a while back from one of the Edinburgh Folk Club organizers asking what Jeff Warner was like - after they'd booked him. (Good decision, as it turned out). I presume they were contacted by an agent, found out that Warner was playing at a number of other venues who knew what they were doing, and just took a punt on him.

It would probably have been easier to find out what Warner's approach to his material was (hard-core traditionalist with first-rate scholarship) than what it actually sounded like (quiet deadpan humour and terrifically effective playing on many different instruments).