... by hovering mouse over dead assembling machine. The tooltip shows available electricity amount for it. That’s enough.
There are million ways to break things in Factorio. Only very few cases are displayed by flashing icon. There’s no notification if machine is out of materials, if output is full, if wrong recipe set, if wrong items delivered, if inserter missing or it’s in wrong position etc. Player needs to check all these things to realize what’s wrong. We all have had hundreds small and big production fixes without any single icon. That was ok for us.
Some small mistakes are more frequent and not challenging at all, so flashing icon is useful (e.g. regular “no power” icon). BUT if there’s a power switch, that means electricity loss is planned by design. If something unplanned happened, player will check it like anything else in his factory.
Nevertheless, I think it’s ok to have some quiet icon “turned off”, which is not a warning, just informing, like “Bottleneck” mod. No annoying flashing.