Do you mean voltage drop or current leak? A combination of the two would then be power loss, which is what I imagine they are attempting to find due to suspicion that they are paying for more power than they are actually using.
I'm sure there are some testing devices for more specific, but using an ammeter and a voltage meter you can probably narrow down a potential problem.
- With an active device, such as a motor/fridge/etc., you could take a an ammeter reading near the device and then back at the panel to see if there is a noteworthy difference, indicating a "leak", or draw, somewhere on the circuit.
- You could take an ammeter reading with the device on the circuit inactive to see if there is a current draw when there shouldn't be (again, take readings near the device and then back at the panel).
- You could simply shut off the main disco and see if the meter is still spinning
- Test voltage on a particular circuit at the panel and then at the end of the circuit run to find a difference
- You again try this with the circuit load activated to see if the voltage comes down under load.
Remember your Ohms law - voltage drop increases wattage at a particular amperage. A voltage drop can increase power consumption.