Q: How do I know the difference

Welcome to the Apple Support Communities

The main difference between a 3rd and a 4th generation iPad is its connector. If you look at the bottom of the iPad, you will see the connector. In the 4th generation iPad, there's a Lightning connector, and in a 3rd generation iPad, a 30-pin Dock connector (as the one that you can find in old iPhones and iPods)

Identifying iPad models

http://support.apple.com/kb/HT5452?viewlocale=en_US&locale=en_US

 Cheers, Tom