I find it most useful to talk to someone else about books. It could be a friend that I know has comparable tastes, so that I just ask, "What've you read lately?" It could be on a web board or on Facebook, in which case I list a few criteria of what I"m looking for so that I don't end up with a birther book.
Outside of this, newspaper and magazine reviews are good for giving some slim idea of what a book is about, even though they generally do fiction and memoir-type books. I regularly check NPR and the New York Times for these.
Finally, I've found Amazon's reviews to be pretty reliable, in the sense that I can tell whether a review is inflated or not.