A good charge on a batter should give a voltage reading in the 13V range with the car not running. When the car is running and the alternator is charging a volt meter will read in the 14V range.
Watch the meter when the car is turned off. The reading should be fairly steady. If it is dropping something is draining the battery.
In the old days we used to just disconnect the cable while in the shade and touch the cable to the battery terminal and watch for a spark. The bigger the load, the bigger the spark. That doesn't work well with todays system because some will draw a large current when reconnected, causing a larger spark.
A decent amp meter is a better way to monitor battery drain with the system turned off. As mentioned previously, removing one fuse at a time can help isolate what is causing the drain on the battery if it isn't a bad battery or charging system.
Beyond this, trying to explain how to measure electrical circuits and explain the problems that can ensue could take a lot of time and effort.
jtk
"A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty."
- Sir Winston Churchill (1874-1965)