You can disconnect a battery and see if the alternator will continue to charge, but it is not worth the risk with the low quality alternators found in most boats. If you attempt it, make sure the battery charge rate is minimal and you have some sort of resistive load to absorb the voltage increase. Do not have any electronics powered as the power supplies may not absorb the spike.
An improved method would be a good voltmeter. Read the DC volts and the AC volts at the generator output. A high AC component indicates a possible bad diode tree. Steve DeLange
