Here's an item that I have added to my urban survival kit. It is a voltage detector that works on the electrical field created by ac current. This is the "Circuit Alert GVD-504A" made by GB Instruments. It's a pen type thing about 5/8" square x 5" long. I think I paid something like $12 for it. You hold it near an energized conductor & it emits a beepbeepbeep sound. It will detect the electrical field in my desk lamp from 8" away.

I normally would assume that downed power lines are live, but in a major calamity you never know what may be happening. If you have a partially destroyed building, it may be useful to know if exposed building wires are live. Again, I say I would stay away from this if possible, but it may be necessary to pass through such an area to escape the debris or rescue someone.