What would be the best/economical solution to detect an AC device activation?
I haven't tried it but I'd be inclined to try a remote current indicator. These are a CT driving an LED and you should be able to either monitor the voltage across the LED or replace it with an opto-isolator giving double isolation between the mains and your microcontroller.



Figure 1. A remote current indicator. (Image deviously obfuscated to avoid product recommendations.)

To increase the sensitivity you wrap multiple turns in the primary of the coil.

Just be careful that the LED in this device isn't a pair of back-to-back LEDs to handle the AC. If you're going the opto-isolator route you could choose one with back-to-back IR LEDs internally.