The traditional method for determining if a motor is running or not is to measure the current flowing through one of the wires supplying power to the motor.  When the motor is off, the current should be near zero and will jump to the running current when the motor turns on.  The classic sensor for measuring AC current is a current transformer.  In a typical AC current transformer configuration, one of the wires carrying current to the motor would be routed through the transformer with the one wire forming a single-turn primary.  The magnetic field produced by the current flowing through the wire induces a voltage in the secondary winding of the transformer, proportional to the current.  The output voltage of the transformer would then be connected to a circuit, which would activate a switch, whenever current was drawn by the motor.  However, this single wire connection is often difficult and can be dangerous for a novice to wire into an electrical system.  The installer would have to cut into the power cable to expose one wire or find an exposed single wire somewhere else, perhaps inside the circuit breaker box.