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)
You can make a UNTUNED energy detector, with a diode, 2 resistors, 2 filter caps (to set the upper frequency limit) and a switch to change that frequency limit.
You'll use a 9 volt battery, powering a series connection of 1Megohm resistor, the forward-biased (schottky) diode, and 10,000 Ohm resistor to ground.
Across the 10,000 ohm is 100pF capacitor. At 1GigaHertz, this has about 2 ohms reactance, thus the WiFi energy (if the antenna picks up any) ripple across the capacitor will be quite small, and your readout device ----- a DVM ---- should be accurate.
To explore Low Frequencies --- Air Conditioner spikes --- place 1uF across the 100pF, but SWITCH the 1uF in and out. Keyfobs may still be detected, and 60Hz or black-brick trash will still be detected.
Build this over a FR-4 board with standard copper foil. At the top of the diode (the shared node of diode's anode (the base of the arrow, not the bar) and the 1MegOhm resistor), install 1" solid_copper wire, approximately 90 degrees from the copper ground plane.
To look for other interference, simply clip some test leads to the 1" wire.
Now your interference may be VERY QUICK. May be tough to catch the miscreant in action.