Hi.
(1) Primarily by listening,
(2) but also measured resistance and voltage drop, as well as looking for ripple on top of the mains voltage with an oscilloscope.
First off, did you audition to compare a few power cords first or did you do the measurement first?
The sequence is
vitally important for one to cast its verdict of the tests.
(1) I hear all cables sound different, including power cords.
To prove my audition is correct, I went thru the 'pain' of comparing 3 types of IEC power cords,
(a) stock power cord got from any electrical stores. Dirt cheap.
(b) power cord terminated with IEC connector/plug, which I DIYed using a Hi-tech power cable, with
anti-vibration filler/jacket, braid shield, specially made in Japap for audio use.
(c) power cord terminated with the same IEC connector/plug as (b), which I DIYed using 99.99%
pure solod silver wires of German origin, no shield.
Plugging to the same tube power amp playing same music, the sonic difference was fairly distinctvie.
(a) cord sounded veiled, slow & lacking details. - OUT !
(b) cord sounded pretty good, transparent & detailed. Good!
(c) cord sounded fast, transparent & very detailed. BEST!

The result pushed me building a proper power cord using #12 4N pure solid silver conductors for my
tube power amp as nothing similar cords available in the marketplace. I just want the best!
(2) What you tested was like micky mouse when compared to a thorough Total Harmonic Distortion test of carried by the chief engineer Class D Audio at Philips Digital Systems Labs, published a few years back.
3 different interconnects are used for the test.
(1) Radiio Shack type freebie come with any DVD/CD player.
(2) a hi-end Japanese brandname cable.
(3) coaxial cable of 50R impedance.
The THD spectrum analysis charts came out to be nearly identical 30mV & 13V feeding test signals with O/P cable load of 20R & 600R. Almost identical for all 3 cables.
So this THD spectrum analysis test proved nothing.
Yet the conclusion of this test was:
"It shows that people who claim that cable do
NOT make a difference asre plainly
DELUDING themselves. Those that say that cables
should not make a difference, are dead right."
From the above test, it is obvious that THD is not the factor that makes cables sounding different. The wrong data was measured which proved null.
So what data should be used to tell the sonic difference of an electronic part/component is still unkown todate.
So until such day arrives in the remote future, hearing is still the best tool.
c-J
PS: Likewise, I built 6 pairs of audio interconnects using same 4N pure solid silver wire of German origin
for my rig. No choice when one wants the best!