I tend to judge them by whether or not they sing folk songs -
for a start.

Then, perhaps, whether or not their performance style is appropriate for their choice of material, and do they display sympathy for, and understanding of, that material?

Do they bring some originality to their performances - or are they just a 'clone' of some other folk world 'star'?

Do I get to hear some songs which are new to me, learn something new about folk music, and/or do I get to see a song (or songs), that I thought that I was familiar with, in a new light?

Finally, at some point in the evening, do the hairs rise on the back of my neck?