How do I tell if a light switch is live?
Head down to the local hardware store, and pick up a non-contact voltage tester (any brand will do).






Carefully remove the cover plate from the switch, and move the tester near each wire (you won't have to touch the wires, just get near them).  If you hear a beep and/or the light turns on, the circuit is hot.

If the circuit is dead
If the circuit is not hot, you still will not be able to cover the box completely. You'll either have to remove the box and all wires leading to it, or cover the box using a blank cover plate.

If you decide to use a blank plate, you can wallpaper the plate itself (while not attached to the wall). This will help it blend into the wall, and all but disappear.
If the circuit is live
If the circuit is still hot, you'll have to trace the circuit back to the point of origin and disconnect it.  In most cases a light will not be the only thing on a circuit, so it probably will not be as easy as removing a single circuit breaker. You'll likely want to contact an Electrician to disconnect the wires, and remove the circuit in this situation.