As a simple isolated yes/no, on/off event - skyhighfallguy covers it. There isn't normally any way to tell (from outside the aircraft) why a transponder has ceased transmitting simply from the occurence itself. It was switched off; it failed; it was broken intentionally; something else in the equipment failed - all will give the same appearance, externally.
Except...
I do see one possibility. Transponders have multiple "on" modes. The most basic is mode "A", that responds to a radar pulse with just a 4-digit "squawk" identifying the aircraft - e.g. 4-4-1-5. Next higher is Mode C, which alongside the 4-digit number, also transmits the plane's altitude. Most complex is Mode S, which transmits additional flight information.
In some transponders, these are controlled by a multi-position rotary switch, and it is theoretically possible (but practically, real-world, unlikely) that an observer - most likely reviewing the recorded radar data after the fact, not watching events in real time - might notice that the transponder changed modes a fraction of a second at a time, from S to C to A to "standby/off" (not responding) and determines from that, that the knob was intentionally turned, rather than the transponder being destroyed all at once.
However, against that, one would have to consider that computerized radars that can read and display transponder data, also usually have a "coast" mode - if they lose a plane's transponder, the aircraft does not immediately disappear. The computer behind the "radar" screen keeps showing a symbol for the aircraft, moving it "predictively" based on the previous speed and direction and last known data, with a color change to indicate the loss of real transmissions. That might tend to "muddy" any data on mode changes just before signal loss.
But hey - no one every claimed Sherlock Homes was true to life in every detail, either.