The reader reviews for the various CLEP and GRE test guides on Amazon.com are plentiful and often a clear consensus emerges. The reviewers have almost always taken the test and will often describe a particular book's strengths or weaknesses. As with all Amazon reviews, the star ratings themselves aren't sufficient (although they can tell you which books to avoid). You have to read judiciously, but it's usually clear who knows what they're talking about. Sometimes a lone negative review is an inadvertent recommendation if the only grounds for dislike aren't relevant to your purposes.
If it helps, I've used the same method for many other purchases (usually technology toys). I've never thought of applying it like this, but it makes perfect sense to me. As you stated, often times a lone dissenter turns out to be a "closet" recommendation if it becomes obvious that the individual has no patience with reading instructions, or no idea what they're talking/ranting about. Thanks for the innovative idea. Dave