There are lots of apps that will show you the signal strength of a wifi signal (usually in dBm, ASU, etc.).  But how do you test or measure the signal quality?  (Ideally without buying any, or much, new hardware.)

My particular use case is to determine how high I can push the wifi transmit power (within legal limits, of course) for various routers sitting around before they start distorting.  Sure, I could just measure data rates, but I was hoping for something quicker and more elegant.

Any suggestions?