how to make a engine sense circuit

I have been searching on google but have not found a way to "sense" when a vehicle engine is running.

I need to be able to take a lead from the battery, tach, or alternator and be able to sense if the engine is running. I think this would be by sensing a alternating current with in the dc power but how to i protect the circuit from the different voltage of a battery or alternator lead versus from a tachometer lead. Would the tach lead not spike out around 200V from the ignition coil?

When the engine is running (alternator running) the voltage (at the battery) will be above 13 volts. When the engine is not running, the voltage will be below 13 volts. You can use a voltage divider, or optical isolator to scan that on an analog input pin.

This seemed like a good answer to me:

http://forum.arduino.cc/index.php?topic=148857.msg1118509#msg1118509

How much do you know about the engine you want to monitor? If the engine uses an electric fuel pump, just tap off the power to the pump to operate a small 12v relay.