Several ways to tell:
- If the car wallows, bounces repeatedly after going over dips or undulations in the road
- If after pushing down on a corner of your car, it rises and falls more than once in each direction (noticeably)
- Leaking, this is not always proof, but often a blown shock will have oil dripping down from lost seals.
- Finally, drive qualities: It doesn't handle well, does feel secure on the road.
It would be amazing if the shocks were that old. They could have been replaced with OEM ones and it just isn't evident.
Since the construction of the shock has separate sections for the gas and oil sections, it could be worn but still have pressure. This is from the Bilstein pages:
"The nitrogen gas is separated from the superior grade oil by a specially designed, close tolerance, floating piston. This design maintains constant pressure against the hydraulic fluid, absolutely eliminating fluid aeration and performance loss that is common to conventional twin tube design shocks. The self-adjusting working piston, with spring-steel valving discs, precisely regulates oil flow for optimum compression and rebound control"
Curious, why did you decided to do the shocks in the first place?
What kind of shocks are you using to replace with?