You can easily learn to test them "on the car," and surely you've read some of my posts.
There are several areas you need to check, assuming factory wiring
1--You need to determine if the alternator output is actually hooked to the battery, IE problems in the bulkhead connector / ammeter circuit on the way to the battery
2--You need to determine if you have a field circuit wiring problem, easily done in the course of tests
3--Determine if the alternator can and will charge by bypassing the regulator and in fact the harness wiring, as in the case of the 70/ later system, it might not be getting field power from the blue field wire
4--If the alternator can be made to charge by bypassing the regulator, then you "add in" the harness, until, finally, the only component you've eliminated is the regulator.
5--Make certain that the regulator was actually grounded, and everything above is good, replace the regulator.
All of the above only takes a few minutes once you learn how. When / if you DO replace an alternator/ regulator after on--vehicle testing, this is no longer pretty much "throwing parts" at the problem, because there is now an almost certainty that you have found the problem.
Of course if you don't mind having a stack of spare parts, or if you DO have, and it's quicker just to "throw" a new alternator/ regulator on the car, then that's OK too.
But what...........do you do...........if...........in that stack of spare parts, it turns out that both your spare alternators are actually BAD? This means you've just changed one or two alternators, and STILL don't know what the problem really IS?
Or...........what happens..........(and it has!!!) if either the "parts guy" doesn't know how to use "the tester" or the tester is just plain broke? He tested the (good) alternator as "bad" so that's what you buy, you drive ALL the freekin' way home, and replace the thing, and ..........well you know.............