A leaking shock is not the only way to tell if a shock is worn and it’s not the only reason to replace shocks or struts. If a shock is leaking, it means the top seal has failed. There are several other components in a shock or strut that affect performance. The inside of a shock has hydraulic fluid, internal seals, and multiple precisely-calibrated mechanical valves. All these break down and wear with use. It’s very possible they are worn and no longer able to effectively control the tire and the weight of the vehicle, while the top seal is still in good condition. It’s also possible for the top seal to wear enough that the nitrogen gas has leaked out, while the thicker oil has not started to leak.
Most shocks and struts should be replaced not because they are leaking, but because the mechanical valves have become weak, and the vehicle has lost it’s designed handling and control capabilities. The best way to determine if your shocks and struts should be replaced is having a qualified technician perform a road test. They can benchmark your vehicle’s current handling and control capabilities against its designed capabilities and provide you with a report.