See post 34 of this thread - http://www.mastercraft.com/teamtalk/...9&postcount=34
As mentioned by robpout, you should hear a definate change in sound in the ballast pump as it goes from pumping water out the bottom of the boat to sucking air from the empty tank. At this point the pump is spinning faster due to the reduced strain - higher rpms of the pump = higher pitch noise. You can test this by filling the tanks for a little (you should hear going from a higher pitch sound to a lower pitch sound once the ballast pump primes), then reverse the switch and when the tank empties (pump looses its prime and rpms increase) and you will hear the sound change again. Once you get to know the sounds the ballast pumps make, you will know if they are empty or not just by the sound.
Also, if you have the pump AI timers/controls (and you should unless someone modified and removed them) the AI timer/control should observe the tank is empty (sensing the amperage change) after a short time (oh 5-10 seconds, maybe a little more) and your pump will automatically stop and the switch should display 1 blink. It is always nice to confirm the tank actually empty by observing the change in sound.
Since you are new, the thread link posted above is the single post view. To see the entire thread it was posted in, click on the "thead" link in the upper right hand corner to read more about the ballast timers.
Also, if you search on "ballast timers" you will find lots of information on this subject.