Let's assume you have the entire factory drivetrain, and you have an auto tranny, then you would have a Turbo 400 like I do.

At the end of the speedo cable, you will see a round "plug" in the driver's side of the tail end of the transmission. That plug is held in place with a metal clip secured with a bolt. Remove the bolt and clip and wiggle the "plug" back and forth a bit while pulling it out. It's a tight fit and sealed with an "O" ring. That plug is the housing for the driven gear. It just sits in the housing. You can pull it out and make sure the teeth are not stripped and the shaft of the gear (that turns the speedo cable) is not broken. A small amount of tranny fluid will probably want to seep out with the housing removed so be ready to catch it with something.

The drive gear is a round "hoop" like structure inside the tailcone of the transmission and cannot easily be removed for inspection. You can see a small section of it with the driven gear housing removed.