Maybe the cap is just worn. It happens, you won't see by eye. The polymer caps are self healing, when the high voltage punches the plates trough a polymer isolates the shorted dots. So after time the cap has no capacitance.
Other types caps like electrolyte dries over time and has no capacitance.
Now, basically you have two types of capacitors: run and starting. The starting cap is engaged by means of centrifugal switch mounted on a motor shaft, when it reaches certain speed it is disconnected. Meanwhile the run capacitor is always connected. All combinations are possible: run, run + start, start.
A higher capacitance will give you extra torque and more current load (motor heating), but if reving at high speeds - ie. motor unloaded it will be destroyed due to high voltage present on the terminals. Lower capacitance may give you not enough torque to start, so that's why an extra run cap isneeded.
It all depends on the type of load - a pump does not need a start cap, but only a run cap because it never spins freely. A circular saw might needs both of them or just a starting one, since the timber shall not be present when starting a saw.
If yo don't own a multimeter with capacitance measurement, then the best thing to do is to buy a new capacitor of the same capcitance, knowing that the load type is still the same. Else you might search a standard capcitance value knowing the rated power and speed, (current,..).
On this link you have all three types combinations and reference capacitances vs. speed and power.