One method we have used to find a short on the old plug style fuseholders is to replace the fuse with a standard lightbulb. If there is a load or short on the circuit, the bulb will light. Start by removing all loads from the circuits. If the bulb is still on, half-split the circuit (pick an easy spot to work on). Keep using this method until the bulb goes out.
For breakers you can install a pigtail socket w/bulb in series.
