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

This is ONLY and IF the rotor and the drive tang are in line.

99% of the time they are not.

Telling people to use the FSM and not telling them that it almost never works is not good.

I’m betting I have 30-40 distributors laying around and not one of them has the rotor lined up with the drive tang.

i've never had an issue lining things up like the service manual.