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Sensing it is one thing, transmitting the reading wirelessly is another.
For sensing you'll have to decide whether you want to verify if it's actually producing any torque, in which case you'd have to measure the rotation of the mechanical axis. This can be done with an optical encoder disk and a photo-interruptor (light gate) for example.
If you're satisfied with an indication alone then you could also measure the electric control signals for the motor, there it would depend which type of motor/engine the car uses (brushless, fossil fuel etc.).
To select the proper wireless interface you have to decide what range you want to transmit your data over. To get a reading on a smartphone you can opt for a Bluetooth/BLE interface. To gather centralized data you'll have to turn to GPRS, HSPA etc which needs a lot more power and more complex hardware.
Finally, for a wireless application, you'll have to think how to power it. Batteries are never a good idea, so either you somehow interface it with the car's power system, or you make use of energy harvesting (heat or mechanical vibrations, both of which are abundantly available in a car) to power the system.
Thanks for your reply. The idea came from an electronic parking disc, that senses when the engine is on. If I could replicate the circuit, it could be very useful in other applications. The manual for the parking disc, mentions 2 sensors. 1 gyro, and 1 motor sense. Just not sure what kind the latter is.