Does a dash mounted voltmeter gauge reading of less than 12 volts indicate that the the engine and accessories are using more electricity than the alternator is putting out and pulling the battery down? I am working on an engine swap and have been starting the engine, shutting it down to work on it and then starting it again. I am having trouble with getting the starter to crank after a few starts. Would the volt meter tell me whether the problem is the battery is not holding a charge or whether the charging system is not charging the battery adequately.

I'm sure there are many ways to diagnoses this problem. Personally I would start at the battary. Make sure you have clean cables and check the voltage at the battary leads. Should read a bit over 12V with everything off. Once started, should read about 13.5V (+/-)

If it's charging OK (13.5V) then take the battery out and have it checked. Maybe a dead cell?

It should be showing more than 12 volts when the alternator is charging but like David said I would check the voltage at the alternator and at the battery myself.