[Kevin in NJ]

Where the teeth engage the other gear they should be a little pointy, not flat.

The teeth were broached and would have straight lines across the face front to rear. If they are shiny and smooth then they are worn.

On the sliding shaft move the gear fore and aft and feel for lip at the edge of the gear travel. That would indicate wear on the shaft.

Shine a light on the oil and look for reflections from metal pieces.

Look at the forks for wear.

As a general rule you buy cheap expecting to replace a lot.