Yes, it's not that hard. There's smart meters and plugs that have frequency measurement built in.
You can even do it with an audio cable: https://halcy.de/blog/2025/02/09/measuring-power-network-fre...
Having the grid operate at 49.99Hz instead of a perfect 50.00Hz for a day will make your clock lose 17 seconds, but it's completely harmless. That's normal regulation, not a gradual failure. The grid chooses to compensate for that by running at 50.01Hz for a day, but that's solely for the benefit of those people with old-school clocks - the grid itself couldn't care less.
A failure means the frequency drops from 50.23Hz to 48.03Hz, probably within a single second. You'd notice as your clock stops ticking due to the resulting power outage.