What are some quick, cheap hacks for determining the source of EMI in my house?
A yagi antenna and a cheap SDR might be good to play around with. Make sure the antenna and SDR work in the frequency range of interest (so if you want to look for wifi signals, then the antenna and SDR need to work in the 2.4GHz range.) MSI SDR is an SDR I like because it has a range of 10kHz to 2Ghz (but you need 2.4GHz). RTL-SDR shows an SDR solution for 2.4Ghz.
SDR's are software defined radios, you can download spectrum analyzers for free and see which bands are carrying radio information.
Keep in mind that you may not be able to find exactly what the source is because of reflections and attenuations of walls and other objects. The antenna could also have a lobe in the back so read up on antenna patterns (or look at the one you buy, antennas are not perfect by any means, they have different lobes and nulls which changes the gain with direction)