In a 12 volt system, an alternator charging the battery will output 14.3 volts. A battery by itself will output 13.6 volts. A better test would be to reconnect the battery, and start the truck. Before performing and testing, rev the engine up to about 2500 RPM. Many alternators need a good rev to start generating power. Once you rev it, just let the engine idle. Place a voltmeter between the two terminal of the battery. If its 14.3 Volts, your charging system works fine, if it's 13.6 or below, your alternator is no good. If it's between the two, your alternator is questionable. I'm sure other members of this board could tell you what the two different test voltages are for a 6 volt system. I do not know off the top of my head for 6 volt.
I think removing the battery with the motor running is a bad idea, as it acts like a large capacitor, and regulates the voltage in the vehicle in addition to the voltage regulator either internal or external to your alternator. Your alternator generates A/C power, which is then converted to D/C power internally. This conversion process is not perfect, and results in a non-constant output current that is sort of D/C. The regulator alone can only lower high voltages. The battery can provide current to avoid low voltages. Basically, the battery is in parallel with this "sort of D/C" that the alternator is generating, and the result of this is a steady D/C power source for the vehicle. Based upon how good the conversion circuit in the alternator is, which will vary from seemingly identical alternators, running without a battery may work in some systems, it may cause the engine to die in others, and in some instances, I think it may damage the rectifying diodes in the alternator.
Doug
