I have a "cellular data" button in Settings on my Verizon 3g. If I click on it, I can turn it on or off and view the account information.
The ATT models have a SIM card slot.
Look at this site ->Apple - iPad 2 - Two great ways to stay connected. and a picture where it shows wifi and wifi + 3g version. You`ll notice black piece of plastic on the back of the 3G model, while Wi-Fi model doesn`t have it.
On top lefnt side of screen it will show a telco whether or not your have regiustered IPad2 with them. If no telco the IPad is a WiFi only.
IPAD 2 Black 16 G WiFi + 3G