When the phase gets pulled down hard like what nearly happened in Texas and what probably happened here, it'll go from looking like the background noise of phase changes to catastrophic in just a few seconds. It isn't like you'll get warning an hour ahead of time. You'll probably notice your computer monitor going dark before your grafana graph refreshes.
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.