[dread78645]

But what I lack is physical confirmation of the length of one microwave, which, as far as I know, is outside the frequency for visible light.

Uhm that's what you're supposed to use the ruler for.

lf = c

l - the wavelength you measured.
f - the frequency of the oven's magnetron
c - the speed of light

If the oven operates at (the standard) 2.45 Ghz you'll see melted/unmelted nodes about every 12cm.

[Fester Chugabrew]

Okay. Does the oven's magnetron generate the same frequency as the speed of light? Also, how am I to be certain what I am measuring is really a wave without taking someone else's word for it?