You can check the model number on the back if the case. Each version of the iPad 2 and 3 has it's own model number. This site lists all the model numbers. http://support.apple.com/kb/ht5452
The easiest way to tell, though, is by looking at the display. The iPad 3 has a retina display and the individual pixels cannot be seen at a normal viewing distance. On the iPad 2, you can make out the individual pixels.