Then it’ll be found and the consequences of having launched a nuclear weapon at the United States will be incurred for no gain whatsoever. That is to say, 0% chance. That would be unfathomably dumb.
In order:
Signals intelligence proximate to the plan's inception that'd likely thwart things before the situation escalated
Monitoring of the incoming track and its electromagnetic emissions
Observation of the blast from a variety of sensor platforms
A WC-135 flying near the atmospheric blast area to determine isotope composition