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

There is no wrong way to install the drive gear; but there is a factory correct orientation of the drive slot for a correctly oriented drive tang.
On some of the Chinese distributors the drive tang is like 90* different from the factory, which then orients the rotor that far out, if the drive slot is correctly oriented.
None of this matters if the wires are long enough to be re-clocked, or unless there is some OCD involved.
The engine will run no mater where the rotor is pointing, so long as at #1 TDC compression, the #1 wire is plugged into whatever tower is directly above the rotor-tip, when the pole-piece is aligned to a vane.