Is there a way to reliably test voltage of a 12V battery when it's not disconnected (i.e., partly loaded)?

I’m having an issue where a fully charged battery doesn’t necessarily report the proper voltage when checked from the socket within the car, because in order to have the socket activated, the car has to be ON, which, in turn, puts non-trivial load that drops the voltage below what should be the proper voltage without the load (as the 2.1V+ per cell rating, 12.6V+ total, is the full-charge at-rest rating of the battery without any recent charging or load).

In fact, I often find that it’s even a problem when trying to measure the voltage of the battery directly from the battery after opening the hood, because the car continues to consume some extra above-idle power until a certain timeout of some number of seconds and/or minutes.

Is there any reliable calculation, formula or a rule of thumb to quickly assess the charge of the battery in such conditions, without fully disconnecting the battery, nor resorting to very expensive specialised measuring tools?

• I’m thinking there must be some known estimates how much a given car is expected to consume when the key is ON, parking break is activated (i.e., all lights are off), radio and inside lights off etc.

• Likewise, there should be some tables on how much the voltage would drop in the situation as above.