The best way to check shocks is to find a stretch of bumpy or wavy road. If you can traverse it at your normal speed without excessive body bounce or wallow the shocks are okay. Some new cars won't pass this test, but I don't own a Buick.
One thing to understand about the high pressure "de Carbon" type shocks that are OEM on Mercs is that they maintain the original damping characteristics until end of life, which is usually indicated by virtually no damping at all and leaking oil. Conventionial twin tube hydraulic shocks slowly deteriorate over their life. It's not unusual to see de Carbon shocks last over 150K miles and when they go, it's usually pretty obvious - either by all the oil or uncontrolled body rebound on the corner with the dead shock.