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

you might check rotor phasing: with the dist in or out of the car with any reluctor tooth lined up with the magnet & JUST starting to move away from it, the rotor terminal should be lined up with a dist cap bulge/terminal or pretty close. you do have some leeway there. put the cap on and make a sharpie mark on the side of the dist at any cap bulge. take cap off & line up reluctor tooth as mentioned that puts the rotor tip close as possible to your mark on side of dist & & see if the rotor aligns with your sharpie mark at the cap bulge. close is OK but too great of distance will cause a misfire. in use only the vac advance changes phasing but you can take the dist apart & alter it if needed. EDIT vac adv will shift phasing ccw (on a small block).