How can I tell if I have bad shocks or bad springs? What are the differences between the symptoms?

I have 79 SA with a GSLSE rear end conversion with illuminas, GCs in front and eibachs in the rear. I need replacment of one or the other and I don't know the symptoms to isolate which is which. Help would be appreciated. Thanks.

 

Then how do you know you need to replace something?

In general springs do not go bad. When a shock goes bad the corner with the bad shock will get "floaty" and not seem to settle down as quickly as it should.

 

I think the only reason to replace your springs is if there is severe sag in them on one side of the car...like my 16 year old stock springs. :P