Is there a mileage marker or car age at which time shocks should be replaced on a car -- or at least checked thoroughly? Or is this something you just determine by feel on the road - i.e. feeling potholes or bumps more and more? I don't do any off roading and am not really interested in modding my car to be something it isn't. The roads here are pretty good with an occasional pothole or bump or two giving my butt a bit of a knock. Short of a trustworthy mechanic giving me sound advice, how can I tell if the shocks on either of my cars need to be replaced? Thanks.