It seems reasonable, as others have noted, that any person who bills himself or herself as a perfomer should be willing to audition for you--or send a recording. You say you don't audition, but you also say word of mouth has not always worked out for you.    20 minutes is in fact too long to sit through a talentless navelgazer. One song--no more-- is about right.

Without some sort of audition--live or recorded (and a recording, as others have observed, may not be an accurate picture of what you will get at your gig)-- there's no solution to your problem. In your position, I'd be willing to gamble if the recording was good.   But an audition of some kind is essential.

Do you pay these performers?