The 12v receptacle will not read the same as a voltmeter placed directly on battery terminals, as there are likely many shared electrical loads present on the circuit which powers the 12v port into which you have your digital voltmeter. But it is certainly better than NO voltmeter, or an analog dash gauge in which one cannot discern a difference between 12.8 and 14 without a microscope.
The voltage indicated does not indicate state of charge, only what the vehicle's voltage regulator is doing as it was programmed to do, possibly with inputs as to battery or ambient temperature and amperage into battery.
My voltage regulator inside engine computer would seek 14.9v after starting and then choose 13.7 in a few minutes, then occassionally jump in between the two when driving.
14.9v on a fully charged battery is too high, 13.7v on an undercharged battery is too low to accomplish much charging while driving.
Now I choose 14.7 or 13.6 and an Ammeter tells me which to choose. If it is accepting more than 0.4 amps at 14.7, not fully charged and I hold 14.7, if it does, 13.6.
i will adjust up or down with battery temperature extremes