The typical "used-car test" is to bounce the bumper a few times, then let go and see how long it takes to settle down. My experience, however, has been that you can always tell by driving. If the car pitches like a boat on the water, or heaves up significantly when you do a rapid lane-change maneuver, it's time for new shocks. Unfortunately, by the time you notice these symptoms, it's no longer safe to drive. All that uncontrolled motion can leave you with no tire traction at the time you need it most, plus the car's direction of travel is always a fraction of a second behind the driver's input. (Then when it does respond, the stored energy in the suspension springs is released suddenly, and you're sideways in the road!)

I suspect that shocks, like all wearing parts, can have wildly varying times to failure, and that the "recommended" service intervals you read are just averages of the entire field. The only absolute is that the longer they go, the closer they are to failure.