The only way to test if a shock is "good" for the average guy is to take it off the sled, take the springs off, and compress the shock by hand. If it does not return all the way on its own, or you can feel spots where the damping changes (air trapped in the oil), it needs to be rebuilt.
As 19 Hundo said, if there is more than 500 miles on the sled then the stock oil has probably broken down in those shocks and needs to be replaced. Most people can't tell if their oil needs changed because it is a gradual decrease in performance. When they are rebuilt and then you ride it again is when you notice that they needed service.