I'm looking into buying a used lift kit and I'm wondering what to look for on it to make sure that it will still function properly. For one, how could I tell if the shocks were bad or maybe even the coil springs?

 

On the shocks, you want to check very carefully for any oil which might have leaked out of the shock. Also look at the rod. It should be bright and shiny and have absolutely no nicks or scratches. That would be death for the rod seal. Beyond that you want to be sure the shocks extend on their own, if they are gas charged and almost all shocks are these days. I'm not sure how to check force values, but if I had a set of four I would compress them by hand and watch them extend, and compare all four for apparently equal force.

With springs about all you can do is look for anything that looks like a hairline fracture. Look for rust that has formed in a line around the wire, which could indicate rust in a fracture. If they are painted, look for spots where the paint might be loose, which could indicate the steel is moving underneath. But it is pretty hard to tell, because paint chipping off is to be expected with springs. Set the springs side by side and make sure they are of equal length. Really it would be pretty hard to tell if something less than obvious was wrong with a coil. If they generally look good, I wouldn't give too much worry to them.