I would just use my iPhone to observe other wifi networks in the area I guess. The world is awash with wifi unless your really out in the sticks. I would certainly hate to have something that was overly sensitive. I think a good visual check for masts etc should be sufficient.
I have to confess though that I don't really buy into this wifi interference thing though. There are numerous videos online showing people flying in fairly close proximity to fairly major installations without any ill effects.