Joe, you basically have to explain to them how to check the oil, and show them where the oil level is supposed to be and where their oil level actually is. But you also need to keep in mind that just because you can explain to the customer what they did wrong and need to correct the issue, you can't make them understand.
Most of the time I have to add the required oil to do any testing to diagnose their issues. You wouldn't believe the number of mowers that I get in that were just purchased that day or a couple of days ago, and have the fuel tank half full of water.