Suppose that you have just been given a second-hand iPad without the original packaging. Apple's iPad comes in two varieties: one with the ability to use 3G Internet access from a cellular provider, and another with the ability to connect to Wi-Fi access points only. If you are not sure which iPad model you have, you can identify it by the unit's physical characteristics or by examining the device's About screen.
Examine the back of the iPad for a black plastic panel running across the top of the device. This panel covers the cellular antenna and indicates a 3G iPad.
Examine the right side of the iPad for a small oval-shaped cover. This cover indicates the presence of a SIM card tray, which only 3G iPads have.
Examine the back of the iPad for the letters "IMEI" followed by a long number. This number---the International Mobile Equipment Identity---is the unique serial number of the device, which cellular networks require. This number is present only on 3G iPads.
Tap the Settings icon on the iPad's home screen, and look for an item titled Cellular Data in the left-hand column. This screen allows you to access and make changes to your wireless account, and is available only on 3G iPads.