how to check a knock sensor

[ricematthews]

I was told once that a decent way to check a knock sensor is to hook up a
timing light to the car and with the car running, tap (not too hard) on the
engine/head with something (i.e. a wrench, etc.).  The way I understand it
is that the light should indicate if the ignition is being retarded as
compensation for the "knocking".  Whether or not this truly works, I don't
know as I never bothered to learn how a knock sensor "reads" knocking.  I
also don't know how this might be complicated by a dual KS engine - but I
suppose you could disconnect one at a time to isolate the other one for
testing.  P.S.:  If you try this, could you let me know if it works?
-Mark