All you can do is measure the current. Up to 50 or 55 mA (that is, .05 to .055 amp) is fine.
I'd guess that if a new battery went dead over one night, you will see maybe 2 or 3 AMPS when you measure it.

CAUTION:
1) Always have the trunk and doors closed when trying to measure the current.
2) Once you connect the meter, the various systems in the car must "die out" after the power is on. SO, connect the meter and ignore the reading for 30 seconds, after 30 seconds the various systems will be "down" and the current you then see is the real standby current.
3) Key in OFF Position and removed for these measurements.