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

Actually, according to the service manual, the slot in the intermediate shaft should point to front most intake manifold bolt on the driver's side and the rotor should point basically straight to the front, when at TDC for #1. I always install that way, because it allows for the most movement of the distributor in regards to the vacuum advance. Read the service manual. That's how you figure it out.