You can find the model number of your iPad by going to the General tab of your Settings app, and then tapping on the About tab. You'll find the model number on the right hand side.
With it, you can "lookup Macs, iPods, iPhones and iPads by Apple Order Number, Model Number, EMC Number, Model Identifier, Serial Number "Snippet" and Intel processor number."
So, you find the model (or serial) number in the Settings app, input it into this site and voilà , you know what type of iPad you have.