How to judge a book by its cover

It may sound sacrilegious, but if you're in a hurry, and especially if you're a fan of genre fiction, you can often spot the sort of book you might like with just a quick glance at the new release section.  (This obviously only works if all the covers are facing out).  Here's a slightly tongue in cheek primer to get you started. 

Action: 
Political thrillers are easy to spot.  Look for flames, stern-looking (usually U.S. government) buildings, flags, and men who have no distinguishable faces but may have guns or briefcases.  Eagles and official-looking crests or seals are another giveaway that there will be lawyers, spies, secret agents, evil corporations, and/or conspiracies. 

Murder mystery: 
Tougher to spot than thrillers, but there are still hints.  Images can be anything, but will usually look darker and more sinister than they would on your average novel.  The background is frequently black, red, or yellow.  Anything police related is a good sign: handcuffs, guns, fingerprints, etc.

Chick lit: 
One of the easiest!  Look for pastel backgrounds with women in dresses and/or high heels.  The chick lit/murder mystery hybrid will often include a similar woman, but will have the aforementioned red, black, or yellow cover.

If you find yourself staring at a cover that does not fit these categories, you're going to have to read the back.  It could be general fiction, non-fiction, or just hiding its secrets.