You probably could not. Cryogenic treatment IME does not significantly change the exterior appearance of treated objects.
Presumably, if you cut sections out of the parts, polished and treated them to bring out any changes in the microscopic structure and looked at them with a microscope, you might be able to see some differences. That's a standard way to make the effects of thermal treatment of metal objects visible, and IME it works. Metalurgy 110.
Interestingly enough, I see people cautioning against cryogenic treatment of plastic parts.
The parts of electrical plugs and outlets that are being treated would then *not* include the plastic parts. IOW, the brass parts would be treated.
This seems to be a relevant paper about cryogenic treatment of brass:
"Abstract. The acoustic eect of cryogenically treating trumpets is inves-
tigated. Ten Vincent Bach Stradivarious B[ trumpets are studied, half of
which have been cryogenically treated. The trumpets were played by 6 play-
ers of varying prociency, with sound samples being recorded direct to disc
at a sampling rate of 44:1 kHz: Both the steady-state and initial transient
portions of the audio samples are analyzed. In some cases, a slight shift of
power in the harmonic spectrum toward the higher frequencies is observed in
the treated trumpets. However, no statistically independent results are seen,
and the most pronounced results were not repeatable. Dierences observed
in player-to-player and trumpet-to-trumpet comparisons overshadow any dif-
ferences that may have been brought on due to the cryogenic treatment. All
data was collected in a double blind fashion. The treatment itself is a three
step process, involving an 8 hour linear cool down period, a 10 hour period of
sustained exposure to -195C (-300F), and a 20-25 hour period of warming
back to room temperature."
The key text would appear to be:
However, no statistically independent results are seen,
and the most pronounced results were not repeatable.
IME measurable changes to resonance are part and parcel of any significant change to the structure of a metal. For example, hardening metal tools makes them resonate audibly differently when you tap on them.
It seems reasonable to say that cryogenic treatment of brass articles may make minimal changes to their metallic structure or no changes at all.
Other sources claim that cryogenic treatment of brass has the effect of relieving stress. This means that their dimensions and shapes are more stable. They are less likely to warp. However, they are talking about changes on the order of thousands of an inch or less. The size and shape of metal parts in an outlet or plug are not that critical. They are designed to flex in use and be self-aligning.