how to tell if the distributor drive gear is installed correctly?

if the eng is running you can check phasing: remove cap & drill a 1/2" hole in the top of the dist cap 2/3 of the way from the center terminal to the #1 terminal. fire it up & shine your light in the hole which will freeze the rotor. rev it & you will see that RPM by itself does not change phasing but vacuum adv in use will. rev it & check the sweep of the rotor to see if it is too far one way or the other from the cap terminal.