Detecting if the power grid demand is above current supply is trivial. Have a look at my power grid monitor experiment to learn how Power Grid Monitor | Blinkenlight. Instead of using LEDs to watch the frequency you could sample it through A/D and then convolute with a 60 Hz reference in Software. This is easy.
However the peak "detector" is actually controlled by the power company. Thus their "peak signal" will not necessarily match what you would detect. I assume that they control for peak price instead of peak load. Although these values correlate they are not always the same.