Amplifiers and current.

There isn't a terribly easy way to do this short of having an amp with that functionality built in, and even then what you will see is a ballpark figure. The problem is this is a moving target in several ways, not only is the power dissipated by the speaker fluctuating with the music content it varies by frequency because speaker impedance varies with frequency, so any method that boils it down to a single number at any given moment can only be an average at best.