Guitar String Replucks
A pluck might produce significantly more broadband noise than a free string.  An FFT of such noise would show more relative energy outside of all the FFT result bins that are related to (F0 or overtones of) a single pitch.  

Also, a free string has a more predictable decay rate in any FFT magnitudes related to the pitch across successive offset FFT frames windowing the sound data.  A sudden change from this decay rate (a stop or a big increase) might infer a disturbance, such as a pluck.