Generic Test with Voltmeter
Before starting the care measure the voltage across the battery. This is your baseline. Should be at least 12.0 volts.
Start the car. Turn on the headlights with brights. Measure across the battery. Should be .8 to 1.5 volts higher than the base voltage. Might have to rev the engine to get the highest reading.
Sometimes on these Bosch alternators you can replace the voltage regulator and fix the problem. This can be done with some cars with the alternator on the engine.