I've found that Bookfunnel is a great place to test a book cover. Granted, you are temporarily giving the book away, but if the clicks are low compared to everyone elses clicks, you have a cover problem. If your "claims to clicks" ratio is low, then you most likely have a blurb issue. If you aren't the promoter of the giveaway, you will have to ask the promoter for the comparison of how your clicks did against everyone elses. But as a promoter, you see all.
Also, I must agree with Chris that, like his book that he decided has no appeal, I ran a small test outside of Bookfunnel for one of my books and concluded from the lack of response that the cover was very wrong. (It had similar problems as Chris') Now, the REAL problem is coming up with the right cover. And, like him, I concluded that spending money on it was a waste until I solved that problem.