I used to test for distributor cap leaks when I was hot rodding by revving the engine on a humid night with the hood open. Humid nights are common here on the Texas coast so I never had to wait for one. If you don't have the pleasure of humid nights, mist a little water around the cap, start the engine and rev it to 4,000 rpm or so. Watch your hands while you are messing about under the hood. The fan is running at night just the same as it runs in the day.
If you have a leak you will easily see it track down the side of the cap. You can see other ignition leaks along your wires and plug caps.