How to use 120VAC as a control source

What's the cleanest method to test if a 120VAC source is on/off?

My first thought is to use the 120V control source to feed a 120VAC to 3V3 Switching power supply, and send the 3V3 output to an Opto-Isolated Relay, then DigitalRead the output side of the Relay.
I "think" I understand how to maintain full isolation when the Arduino is used to control the relay from several posts here, but my concept is actually in reverse (Arduino measures the Relay Output for On/OFF, not controlling the relay as usual)

I feel like I'm missing an obviously easy & safe way for an Arduino to indirectly sense if Voltage is present on a 120VAC source.

Thanks in advance for answering what is probably a stupid question.

AC optocoupler with appropriate current limiting.

"...test if a 120VAC source is on/off?"

A loop or two of wire around one of the AC conductors. An EMF will be induced in the loop/s that can then be read.