[Silicon]

The simplest method is to connect a known resistor (i.e. 10kOhm) in series with the capacitor and apply a small voltage (i.e. 9V).
If the capacitor leaks, a small current will pass through the resistor and this will generate a small voltage.

This can be measured with a digital voltmeter and the current can then be calculated.

Some capacitors may only break down at higher voltages.
You need a variety of voltage sources and resistors to safely test a 400V capacitor for example.

The capacitor could be a dead short and be capable of passing large currents.