The strongest sign an attack is coming that I know of is firm evidence that Russia or China is evacuating her cities.
Another sign that would get me to flee immediately (to a rural area of the US: I would not try to leave the country) is a threat by Moscow that Moscow will launch an attack unless Washington takes action A (or stops engaging in activity B) before specific time T.
The strongest sign an attack is coming that I know of is firm evidence that Russia or China is evacuating her cities.
Another sign that would get me to flee immediately (to a rural area of the US: I would not try to leave the country) is a threat by Moscow that Moscow will launch an attack unless Washington takes action A (or stops engaging in activity B) before specific time T.