It wouldn't measure it directly, it would (admittedly, very hypothetically) be able to measure a knock-on effect of the mains voltage change via secondary effects: i.e. CPU/GPU frequency changing, PSU fan speeds changing a very small amount from the expected rate, maybe even the motherboard / CPU socket voltage amount directly changing (some motherboards expose this info directly for overclocking use).

Obviously, given variation in these things anyway (i.e. due to temperature, and PSUs being in the way), it's incredibly unlikely to be that easy to pin down a robust communications method, but again, might be just within the bounds of possibility.