To determine if your BlackBerry is "unlocked" for use on carriers other than for which it is branded:
1. On your BlackBerry, go to Options > Advanced Options > Sim Card.
2. At that screen type MEPD (see note below) on your keyboard. A new menu will pop up.
If your BlackBerry has a SureType keyboard (71xx and 81xx devices) you will need to double-tap the P, so the actual keys entered become M E P P D.
3. Look for Network in the list.
4. If your device is "unlocked", it should say Disabled or Inactive. If it says Active, it's still locked to that carrier.