Put your DMM inline with the battery and measure amp draw. Do not attempt to start the car in this configuration of course. That reading will indicate if the battery is draining via the car.
Alternatively....You can remove the battery from the car, charge it up to 12.68v then let it sit overnight disconnected. In the AM check and ensure it still is above 12.6v at the very least.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k