Except for the brushes and the wires connecting the brushes to the regulator there is nothing you can test. If you suspect its faulty, just replace it and see what happens.

First of all you should test for electrical leaks.
connect and ampere meter to the battry wire and see is something is drawing electicity. Start with everything off, then swith on all electrical things (including AC) and see what happens. The ABS light comes on if the voltage drops below 12 Volt, that means you are draining your battery. If you buy an ampere meter make sure it can test DC current, most cheap ones only test AC current. For these test (expet the test with everyting off) you will need an clamp-on type of meter.

With a good ampere meter yo can also test the current of the starter. If you can jumpstart it I don't think there is anything wrong with the starter.