This sounds rather like you have a real world situation with a product, and
using the hypothetical toasters as an example :)

I am not sure you will be able to do the testing without unwrapping them,
but after that using a cable finder inside the toaster slot and testing for
the signal on the power plug would seem to be the easiest way of doing it.

Another method that just may work. Many current model DVM's have a
capacitance range on them. Using one of these you could get some calibration
values off units that do work satisfactorily for capacitance between
phase/ground and neutral/ground. Each depot that has stock should be able to
obtain locally a similar suitable meter and do the testing with a limited
amount of knowledge, and so should be able to divide their stock into three
piles on the basis of different readings, even if they do not have an
identical meter. A list of readings faxed back to base should allow someone
there to make a definitive decision on which are OK.

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