Did you know that they have suck a thing as a dyno for suspension? They're called shock dynos, although the name is a little misleading as a shock dyno are equally capable of dynoing forks too. Like anything, there are different brands and methods.
The one I've seen most often is a 5VS made by a company called Roehrig Engineering. It's a 5hp, multiple stroke, variable speed system, which simply means it's capable of doing most things.
Roehrig Engineering's most popular model, the 5VS, is a fully computer controlled, variable motor speed model. It is capable of testing at four specific English or Metric stroke settings, up to 50mm (2 inches). The SHOCK™ Test Control and Damper Analysis software enables you to perform static and dynamic gas tests, temperature or time based warming of a damper, as well as stop the 5VS at bottom dead center of the damper stroke. A standard 5VS has force capacity up to +/- 1134kg (2,500 lbs.) and damper velocities up to +/- 990mm (39 inches) per second with a 50mm (2 inch) stroke. The way it works is simple, but the system is very clever. Essentially the dyno measures damping force at different velocities. Remember, damping only cares how fast it moves, not how far. So to dyno a shock you'd remove the spring and fit it in the machine. One end is fixed to a load cell to record the force, the other end is fitted to a actuator.
Switch the machine on and once it's got the unit up to temperature, it'll go though a test routine measuring the force and many different velocities by extending and compressing it at different speeds. This can then be plotted out on a graph, so you can check for inconsistencies and performance.