If it has no camera or Siri, its iPad 1. If it has camera but no Siri, its iPad 2. If it has camera and Siri and has a 30-pin charger (broad one which plugs into the iPad, its iPad 3. If it has a lightening charger (small size, about 0.7cm x 0.25cm) with both camera and Siri, its iPad 4th Gen.