leakage current can be determine through the use of clampmeter or ohm-meter.
You can use Ohm's law (E=IR) to get this answer. Disconnect power to the circuit, and measure the leakage path's resistance (This is usually the rated resistance of the insulating material...you will need a M-ohmmeter to measure insulation resistance, as it will be MUCH higher than a standard ohmmeter can measure). Then rework the formula to solve for current:
I=E/R
or simply: divide voltage by resistance.
You can also use an inductive current clamp as mentioned above. This clamp measures electrical current by measuring the electromagnetic field strength of the current flowing through the wire.