There are circuit designs that reliably trigger to indicate actual engine run based solely on monitoring the 12V electrical system..
Here is an example of a product using this circuit design..
Per installation instructions and good practices for heavy draw radio equipment, these devices are powered directly off the vehicle battery via an appropriately fused direct connection.
These are used extensively in emergency service vehicles to power mobile radios and other equipment that needs to operate in ignition off conditions, yet automatically shut down the equipment after a user selected amount of ignition off time to save the battery..
I have one of these installed in one of my vehicles. Per documentation, the unit I have monitors not only voltage level (which would be fooled by external chargers) but also monitors the AC noise profile of the running alternator for more accurate determination of actual engine run..
I also use "battery minder" type chargers on this vehicle due to long vehicle idle periods and also due to increased parasitic draw of some additional onboard equipment. On rare occasions the charger will keep the charge guard triggered, but it is very rare..
If you are trying to implement a totally self contained monitor device without additional wires to oil pressure sensors, etc. you might look into the design these devices use. You might get some insight from reading the abstracts of the patents listed...