I had a microwave oven that consistently stopped my Netflix videos streaming over WiFi every time someone made a cup of tea.
I was able to prove a contributing issue was related to its poor door seal letting microwaves out using the free WiFi tool NetStumbler (Also known as "Network Stumbler").
NetStumbler has can graph the Signal/Noise ratio of a WiFi station over time. If you put a laptop running NetStumber in a microwave (Don't turn on the microwave!) you should see the signal to noise ratio drop 30 dBm as the door shielding attenuates the WiFi signal. If not, you probably have an old oven that has developed a wonky door seal.
In my case, I was able to feel the microwave door close a little more as I pressed the handle. And after alternating pressing and releasing the door without changing my body position, 10 seconds on 10 seconds off, I was able to clearly see a 5 dBm difference in the WiFi signal to noise ratio on my old oven. That didn't happen on my new oven.
I also saw other people comment that if a cell phone rings inside a microwave, then that's a sign the microwave is leaky. I doubt that's reliable, since many cell phones use a different frequency than microwave ovens. And they don't report signal strength accurately.