Obviously this isn't a precise way of measuring, but if you're lacking a gauss meter, or just want to cheaply investigate the EMF in your home, this is a fun solution.
Step 1: Putting It Together
Plug the mic into the input jack of the amp.
Step 2: Using It
Step 3: Example
Check out the video for a live demo of what you can hear.
[Sorry about how dark it is, light receptor on my camera's burned out].