| You can have them put on a shock dyno to test the damping and seeing if the damping is not only OK but also matches the spring rate.
If you suspect the damping has failed remove the spring and try pushing in and then pulling out the damper rod by hand at various damper settings and see if things are altering as they shoud do, that is the more you turn the damping up the harder it should be to move the damper rod.
Alternatively just fit it to the bike and set sag etc set damping to minimum and bounce it up and down then increase the rebound until the shock feels as though it no longer tops out. Final adjustments to the rebound and adjustments to the compression can then be made after you have ridden the bike.
How come such a big investment I usually pay £250 or less for a damper like that. | 
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