Pinging weather stations should be a good indicator. If you notice a bunch of contiguous ones no longer responding, or sending back huge temperature readings, there's a good chance a nuclear apocalypse is imminent. (Just ignore the few statistical outliers: https://www.cnn.com/2026/04/23/europe/france-weather-sensor-...)
Except that taking out fiber won't generally result in a roughly circular shape of lost signals, with the ones still remaining on the fringes giving readings that off the charts.
Or that an excavator took out some fiber.