I find that the best test is starting the car on a cold morning. If there are any signs of struggle I take it to my local battery dealer and let them use their fancy tester.

A really good one is going to run you somewhere in the range of $500 to $1000 or more.

The cheap ones that sell for $50 or $60 don't really analyze the battery in any meaningful way, they just look at the voltage and internal resistance and estimate battery life according to a lookup table. Of course that is not always accurate.

I use one of those cheap electronic testers in combination with an old-school load tester which combined give a pretty good indication of what's going on with a battery. (Back in the day a battery hydrometer would also be used to check specific gravity of acid in the cells but these days most batteries are sealed.)

Don't forget that in addition to alternator voltage you also should test for excessive ripple. Some of the inexpensive battery testers will do that but you can also use an oscilloscope to check the waveform being output by the alternator.