I have a light switch that I want to get rid of. I don't know if it is active or not. I want to wallpaper the wall. How do I tell if the circuit is "live" and if so how to turn it off so it is safe to wallpaper over.
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.
Depending on where you are located, it may be a code violation to cover it over completely. The accepted solution in most areas is to buy a blank outlet cover, like this:
Turn off the breaker (and if you can't figure out which one, just turn off the main breaker/fuse and disconnect the entire house)
Disconnect the switch, and put wire nuts and electrical tape over the loose ends.
Wallpaper first, and then put up the blank. If you need to, you can wallpaper the face plate separately before putting it up.