I've been looking for a low voltage, digital way to sense if an 120VAC device was turned on or not. 2 ways to do it: measure if the device is drawing current or measure if there is AC voltage applied at the device. I don't have access to the voltage at the device, so voltage measurement won't work. Measuring if there's current will work.
There's a great way to measure AC current. The sensor doesn't need to touch any AC voltages and it needs no external power. There are these donut-like packages where you run one leg of your AC circuit through the donut hole, and they open or close an isolated set contacts if there isn't or is a current running through the donut hole. They don't require an additional power supply (run off the AC fields generated by the wire), and they don't touch the AC voltages, so your sense contacts are electrically isolated from the AC. There are lots to choose from.
current switch core
on the web. Current switch finds them, but you want the donut style ones (which have a magnetic toroidal core). It also finds some other things, so look for donut like objects in the pictures.
There are many different sizes, current thresholds, split or solid cores, max currents to choose from. For my application, which is measuring on/off 1-5A currents, there are devices available for ~$20. You have to hunt around to find the less expensive ones. Look in the usual spots (Amazon, EBay ...).