Test wires using magnets?

What you are looking for is a "PowerProbe"-can't remembeer all the details but you should be able to google it. Seen it on a couple of car shows...

X2

watch the video at the 1:28 mark
[ame="http://www.youtube.com/watch?v=94npWntQY2g"]Power Probe 3 Demo Video - YouTube[/ame]