How do you tell when a Miata MX5 needs new shocks?

 

Generally, shock absorbers have weakened at around 50,000 miles. That doesn't mean they are shot, just that they are weakened. If they are REALLY bad you can start bouncing the car and when you stop, it will keep on bouncing. Shock absorber is actually a misnomer. They are actually shock dampers. The tires and springs actually absorb the road shocks. The "Shocks" dampen the bouncing. They wear out gradually so you won't notice how worn they are until you replace them.

 

Gas, which is pressurized act differently than hydraulic dampeners once an industry standard. The only absolute way to high pressure dampeners removing them and feeling for the resistance by compressing them and watching how they return to full extended length . And then the 2 inch push and pull the shaft in to the main body. If both compression and rebound move the same the dampener is bad On a hydraulic fluid filled shock the compression and rebound is pretty much the same. However of high pressure gas struts and shock it is different. Mainly because many have internal valving in the main body tubes that slows the rate of the gas and fluid. Some think that the easy compression over rebound if an indication of a worn dampener. The old school "bounce" test.
On vehicle's with high pressure struts and shocks you really need to measure the vehicle's ride height and then push down on a corner and release it allowing the dampener to return to its current state of neutral position. If any dampener is worn out your ride height will measure differently on each front and rea axles.
Natually if there is any sign of physical damage or fluid leaking it should be replaced.