What's the secret to know if and when the shocks &/or springs need replacing front and rear??

Well, there are a few ways to tell.

1. if the car is sagging

2. If you see that there is fluid on the outside of the strut

3. If you push down on the rear or front of you car and it bounces

Those are the obvious ways to tell

The front shocks on my 71 were bad and they failed by freezing up. When at rest, the shocks were stiff when I pushed down on the fenders. When exercised repeatedly or when driven, the shocks would freeze and would not move at all.

As Jon says, if the car is sagging in the rear, the springs are shot. The original springs were good for maybe 15 years at best. You could tell they were shot when the coils went banana shaped and could no longer hold themselves straight. The springs control the rear ride height of the car, so if the rear is lower than the front, the springs are likely no good.

The shocks control the action of the springs. If you bounce the car on one corner the car should not jiggle but should return smoothly to rest position. If it does not bounce, then they are seized. You should only consider Boge, Koni or Bilstein.

Really the only way to tell if a shock is bad on a 914 is to take it out.
Compress it and extend it. if there is a "skip" in the tension/friction when you change directions the shock is going bad. this little skip is detrimental in keeping the tire in contact with the road.

If you can push and pull the piston with little resistance it is ready for the trash pile. Unless it is a high end shock that can be rebuilt.

Most 914 suspensions are so stiff that pushing on the fender only moves the car 1/2" and tells you nothing.

Ive noticed that if it feels like your are driving a pogo stick then you need new shocks

Or when you score a deal on Koni's

Then of course you need new shocks.

It's an easy enough job (and not too expensive) - you should just go ahead and do it. I did this summer, and while I didn't think the old shocks were that bad, putting in new ones made a tremendous improvement!

I wish I'd replaced them sooner!

Hell ya, I had stock to KYB (shit) to Koni reds to my current Biilys on the SC 5 bolt convert all in 10 yrs.. I second the hard to tell with stiff suspensions...mine moves 1/2" if I jump on it...