Looking for pointers on checking ADS damper valves
 

I am wondering if the ADS damper valves in my 95 S600 are functioning properly. I have no dash warning indicators and there are no stored codes. The ride feels too firm, especially on colder days. Some days the ride feels softer, on other days harder. I replaced the pressure reservoirs this summer, they were bad. I'm wondering if the valves may have FOD (foreign object damage) in them. Does anyone have a method to test them for proper operation while they are in the car?