Many of the new testers will fail a unit if the voltage is outside OEM spec, meaning that if your aftermarket regulator charges at 15.2v cold and settles to 14.8v hot that it may show a regualtor failure if the spec allows for a max of 14.6v.
Most of the aftermarket test equipment can only pull about 200a from their load bank or their meter won't read high enough to show true output. While you may not see the full output on a test bench that does not have the full load capability. Remember that the amperage output will not reduce unless there is a failed part such as a diode or worn brushes.
I'm keeping this very general, this is more complicated than I am explaining it here, but the general information is really what will serve you best in testing your unit.
There are exceptions to this, but if you have your system connected in your vehicle you only need a couple tools to test your alternator charging output yourself. The DC amp clamp isn't cheap, so you might want to borrow one, but any audio enthusiast should have a quality voltmeter.
Using your system in place of a load bank may not max out your alternator output, but you will be able to see the output that it is producing, and using the voltmeter you can monitor voltage to determine if there is in fact an alternator problem.
We have a 400a load bank in our shop that we use on our test bench and a 400a amp clamp to read accurate amperage output. If anyone needs a unit tested we can do this for you. You pay the shipping and I will test the output of your high output alternator if you can't get it done locally.