bounce on the car (carefully, without denting it, obviously).... if the car carries on boinging when you stop bouncing it, the shocks need replacing!!!
a bouncy ride is a sure sign... also you could test them by pushing down on the corner of the car. If the car bounces more than once before settling, shocks should be replaced.
If you have a quick look around the forums using the search function, you'll find plenty of recommendations for shocks. If the springs on your car are original, then it'd be well worth replacing those too.
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