There are accounts on Twitter that attempt to do this, down to recording encrypted messages and using the existence and pattern of the messages - rather than the text - as a potential clue.

The various doomsday planes regularly take part in nuclear exercises - which are treated as signals for both friends and foes - so there's quite a bit of data to keep.

Conclusion: unless a full first strike is scheduled beforehand and you're high up enough to be in the loop, you're very unlikely to get much notice.

I have little faith that a compound in NZ or HI would be much use unless you move there well ahead of any Event.

And possibly not much after, because with supply chains gone you're going to start running out of essential spare parts and consumables (including medical supplies) within a decade at most.