The Columbia Journalism Review has some helpful tips for sniffing out hoaxes:

The audience received a thorough primer on forensic image analysis techniques. Recalling doctored images from “superstorm” Sandy and a hoax video of an eagle grabbing a baby that went viral in December, Farid and Clinch walked through methods and tips for quickly identifying phony images. Reflections and shadows in photographs have to line up according to basic optical principals, for instance. Another rule of thumb: Any picture with an unusually placed shark is fake.