I don't think by standers on the ground can discern with any significant level of accuracy wether a piston engine has quit or if it was running at idle, but FWIW I did have an engine quit on me in a DA20 while I was demonstrating spins to a student. Even though the engine came back to life as we recovered, I always found it unsettling that the engine would quit during a spin even with the pumps switched on.

My condolences to the families of both pilots.