Running around for more supplies after an emergency (of any type) has begun is not in my plans, but to answer the general question anyway: I think it’s fairly safe to assume that localized power outages will be temporary, and the larger the area effected, the longer (up to “forever”) it’s likely to last. If you (or any neighbor) can still get a cellphone signal, the internet is working, or NOAA weather channels are still transmitting, those are signs that TEOTWAWKI has not come. When there’s a total communications outage on top of a power outage, it’s harder to tell. I don’t think there’s any single “key” to look for that would confirm that yes, this is “it” beyond a doubt. Mostly it would be a matter of remaining calm, and waiting to see how things unfold over the coming weeks/months.