{1} Can they play their instruments ok, were they in tune before they got on stage, did they have their instrument ready prior to playing, did you tell them when you expected them on stage ?
{2} Do they sing well? They don't have to read the lyrics off a sheet?
{3} Have they got entertainment value? This is the important one - your adience deserves a good act.
It's about that simple as far as I'm concerned - If they do all these things, ok, think about it and get your diary,check they can do a full night's entertainment & book 'em.
40 years and 9 records of experience talking at this point.