I have the WiSpy, a cheap 2.4 GHz RF spectrum analyser and it will
show any microwave (or network or wireless camera) that's in use near
by.

I found an image on their site that shows what it looks like:

http://www.metageek.net/files/images/chan31-forbob2a.png

You can usually move your network to a channel along side your microwave.
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