Q: how can i tell if my ipad is wifi + 3g?

See if you have a Cellular Data setting in the settings app in the list on the left side of the app. If you do, you have a cellular iPad - 3G or 4G.

Only the iPad 4 and the Mini can be 4G.

You can type in the serial number (Settings > General > About > Serial Number, or from the back of the iPad) in on this page and that should tell you info about the iPad : https://selfsolve.apple.com/agreementWarrantyDynamic.do

Or do you have a Settings > Cellular Data option on the iPad - that only appears on cellular models. As you posted from the personal hotspot link, then that is only available on the iPad 3rd gen (cellular models) onwards, and then only if your carrier supports it (some carriers only support it on iPhones).