The deCarbon type shocks (single tube high pressure gas) used on most Mercs are very long lived. Unlike twin-tube non-pressurized shocks that typically slowly loose their damping characteristics over their life, deCarbon's typically maintain their damping characteristics until end of life, then completely fail. This is often indicated by a major loss of oil from the shock.
The best way to test shocks is to find a stretch of wavy road and traverse it at normal speed. If the body motions are reasonably well controlled, the shocks are probably okay. If it wallows around, the shocks could be bad.