When I first read the problem, I thought the task was to detect that the electromagnet was "ON". Detecting voltage applied to the UCS terminals is undoubtedly good enough. But if you want to detect that current is actually flowing thru the electromagnet a Current Transformer (about $1-2 on eBay, free shipping) plus a couple 5 cent LEDs is an option.
Here's a 1000:1 Current Transformer plus a couple back-to-back LEDs on the CT output. A wire with the uncoupler current runs thru the donut hole of the CT. The few Amps of uncoupler current when active is transformed to a few millamps (1000-to-1 ratio) of LED current on the CT output...plenty to light up LEDs. The LEDs are placed back-to-back so that each lights up on the opposite half of the AC cycle and also serves to protect the other LED from excessive reverse voltage.
And here it is in action.
There are other ways to detect AC current like putting a series diode or resistor shunt to generate a voltage when current flows. No matter the method, detecting current (rather than voltage) provides arguably more confidence that the electromagnet or load is actually active.
