THis is how I do it:

Engine off, car sitting, battery better be 12.2 volts or higher. If the battery is over 13.5 volts, it is being overcharged badly. If under 12, it is bad or is not being charged properly.
Starting engine (you need a helper for this) voltage across battery will drop but better not drop below 10.5 volts.
Engine idling should yield a battery voltage of 12.5-13.8.
Engine at 2000 rpm (you need a helper for this) battery voltage should be 13.4-14.4.

Moving the convertible top, raising and lowering windows, or having the engine cooling fan running at high speed will lower voltage quite a bit - but if none of these things are happening, the above readings are pretty close.
For it to be a bad battery/alternator, you will see obviously different "engine off" and starting voltages at the least.
YOu are measuring directly across battery terminals.

I hate how they have removed the voltmeter, oil pressure gauge (and in some vehicles) the temperature gauge.
I have all three on every vehicle I own (Crossfire, Bike and Ranger) and can see trouble coming.