There are many ways to find the capacitance of an unknown capacitor. If you have a capacitance meter then it is pretty much straight-forward but if not then other equipment can be used such as an oscilloscope and function generator. Baring that, you can charge it up/discharge it and plot its voltage vs time to calculate its capacitance.

Example: Let Tau = RC and establish that a capacitor is 99% charged at 5 x Tau

Put a high impedance voltage meter across the leads of the capacitor and make sure the initial reading is 0v. Put this in series with a resistor of known resistance (say 100k ohms) and connect this to a power supply of known voltage (say 5v).

Switch this circuit on and record the voltage across the capacitor every 3 or 5 seconds until it is about equal to the voltage of the power supply. Let Vc be the voltage across the capacitor and Vs be the voltage of the power supply.

Draw a graph of Vc against time. When Vc = Vs then the corresponding time is equal to 5 x Tau. Solving for C using the equation Tau = RC gives you the capacitance. Capacitance thus equals time (when Vc = Vs)/(5 x R)