How to go about sensing AC power in existing wiring
I don't believe there is a cost efficient wait to sense AC power in a pair, no.

Cheap home energy monitors use a lump of ferrite and a coil to clamp around a single wire in the fuse box.

All of the monitors for single appliances I've seen have to be plugged in between the appliance and the wall.

Eg.

http://www.automatedhome.co.uk/New-Products/AlertMe-Add-More-New-Features.html
http://www.vesternet.com/Technologies/Energy-packages/Z-Wave-Energy-Starter-Pack-BASIC
http://www.vesternet.com/Technologies/Energy-packages/Plugwise-9-Piece-Starter-Kit

One possible approach is to take a reading from the main power meter, then try to identify and isolate your target device.

http://www.georgehart.com/research/nalm.html