The Tunguska event could be mis-interpreted as a nuclear strike if it were to happen today over a populated area.
I thought nuclear strikes were highly radioactive. That and other clues would be easy to gather very quickly.
What do you mean "very quickly"? By "very quickly", do you mean "Mr. President, we believe this to be a Russian first strike, and you must decide in the next 5 minutes if we are going to retaliate" quick?