If all you want to do is detect that there is a series of pulses then all you need is to save micros() when each pulse happens and if the time between pulses is too long then you know the device is off. Something like
if (digitalRead(pulsePin == HIGH) {
latestPulseMicros = micros();
}
if (micros() - latestPulseMicros >= maxInterval) {
// the pulses have stopped
}
It might be neater to use an interrupt to detect the rising edge of the pulse - but probably no necessary.
...R