Having had customers that build automotive and motorcycle/snowmobile dampers, and having bought (and raced) a few myself ...
Destructive testing is never part of the production process. Parts go through inspection stations before they get assembled. The critical parts of the process are automated ... if the shim-stack station runs out of a certain shim, it stops and turns on a flashing light and a buzzer. Oil is pumped in using a metering pump that logs the correct quantity. At the end of the ilne, the run-of-the-mill dampers go through an automated function-check station that cycles them a couple of times to make sure the force in response to a certain actuation speed is within bounds. The high-end dampers go on a shock dyno.
If you work for the same facility that builds the dampers, go down to the production floor and see what they're doing. If you are an end-user who is subcontracting them out, make some phone calls and arrange a site visit.
You can't solve a problem that hasn't been defined, so you need to define it. "How to cut the shock absorber" begs the question WHY you want to do that. "To check the dimensions inside" - WHY do you want to do that (and go down to the production floor and measure the parts before they get assembled and there's your answer). "Because we are getting inadequate performance / leaks / premature wear / ???" - WHY is that happening?
It may turn out that you do indeed need to cut one apart - but the method by which you do so, will depend on what you are trying to establish by doing so. You also need to establish exactly WHERE you need to cut it apart.
Waterjet will do it with minimal damage - but it's not good if you want to know how much oil is inside it. A hacksaw will do a messy job - but if you cut it apart at the opposite end from where the dimension is that you are interested in, maybe it doesn't matter.
