How do i know if i have to replace the springs(coils) on my car?

My question is how/when would you know when you're supposed to change the springs (coils) on your car? How do you know if its the shocks or the springs that you have to replace???How do i know if i have to replace the springs(coils) on my car?
Sometimes just one corner of the car is sagging and you need two new springs on the front or the rear. An alignment shop will often turn up bad springs because they can't align the car properly. Springs support the body off the axles, but shocks dampen out the spring oscillations.



http://www.hunter.com/pub/undercar/4187T



http://www.carbibles.com/suspension_bibl



http://www.jcwhitney.com/AIR-LIFT-1000-AHow do i know if i have to replace the springs(coils) on my car?
springs older than 40 yrs maybe time to change if they sag shocks go bad springs not many times at all in a life time shocks bad after about 60,000 miles most of time is what they say easy to look at any if they wet they bad if car has over 60,000 shocks bad
if your car is not saging don't worry about