There is no way to know - no matter how much they advertise- who is going to come. Some things I do- I ask where do you advertise, how many years have you been doing this, how many tables do you have sold...
if the show stinks- I dont do it again the next year....
Some of my best customers now (friends) I have met at crappy shows.- go figure- maybe more time to chat and get to know one another.
I also print out a sheet of every place I will be and put it in every bag. I have had some people follow me from show to show. I put on craigslist, local channels,anywhere it is free. I know its not my job, but I want the sales, so I do a little work too.
I hope this helps...to make you feel a little better maybe- I have done 5 shows this year and averaged $120 (table was 50) so $70...when the same shows last year were 350- 400..I guess you never know- but now I have a ton of stuff for gifts, and a head start on next year.