Hi Steven,
I suggest you to start with the circuit first. Use step down transformers and rectifiers to reduce the voltage from 0-220V to 0-5V level. Use a filter capacitor to avoid ripples. Yes the circuit is similar to a normal battery eliminator, I’ve tried the same and it did work.
You can read this voltage using the ADC of a microcontroller and compare with a threshold value to detect a power failure.