Verify an alert like that? Serious?
There won't be a way in time. There's no "Nuclear War: Real or Fake" real-time website. Phone lines will be jammed, etc.
I guess I must be in the minority to have grown up in a place where signs that said "Fallout Shelter" were popular.
In between Reagan baiting the USSR, Iraqi bomb scares during Desert Storm, and the original World Trade Center attack, I've been through my share of bomb scares and the resulting evacuations or lockdowns.
It's mostly going to be the older guys here who can tell you what us was like to live in a world that seemed a hair away from ending in a flash of light, but no, verification of that sort of thing is what happens after you have taken cover.
Treat it like a fire alarm, regardless of this incident.