Thirty years ago we had equipment in the shop to test starters. Now, since the labor costs more than the new/rebuilt starter, if you take it out, there is no point in testing it because you would be silly to go to the trouble to put an old starter back in.
The on-car test for the most common failure mode in a starter is: When it fails to crank, smack it with a broom handle or similar. If it cranks after being tapped, it is shot, replace it.