If you put your ear against the transducer, you should be able to hear a faint clicking as the instrument sends pulses. You can also use a short-wave or medium-wave radio set to a harmonic of the instrument frequency; hold the radio close to the transducer and listen for a clicking or buzzing sound.
The AD2CP family of instruments all have a blue LED embedded in the head that signify the current instrument state. You can also listen to the transducers by putting your ear against them.
Off: instrument is in standby mode or not connected to power.
Steady blue: instrument is in command mode and connected to power.
Flashing blue: instrument is measuring (flashes at the sampling rate).