There is space surveillance network tracks things in orbit. I don't think that covers all of the sky and could miss a rock. There are early warning satellites

There is space surveillance network tracks things in orbit. I don't think that covers all of the sky and could miss a rock. There are early warning satellites that look down for ballistic missiles. I don't know if the infrared detectors would see the rock. They are probably best as negative signal with no rocket being detected. The main signal that wasn't nuclear weapon would be the size and location. Nuclear weapons don't explode as high as Chelyabink meteor. Nuclear weapons aren't as big as Tunguska event. Also, hitting a city is unlikely, hitting one dead on is really unlikely. There would also no nuclear fallout detected.