I teach digital photography, and usually during the course of the semester, several students will try and pass-off an image downloaded from the internet as their own work. Solution? upload the image in question to: TinEye.com and within seconds, the internet source of the image is revealed.
BEFORE I resort to using TinEye, it is very quick to check for the camera settings using Adobe Bridge...just look at the data in: File Properties, Camera Data, and IPTC Core. If those windows look fishy or a devoid of any information, then it is time to do a TinEye search.
Can be used for graphic designs, too. Sneaky students, beware!