How can I measure if there is leakage current of PV module?

Step1:  You need have one Muti-meter can measure the voltage up to 1000V ( 1500V for 1500V system) .

Step2: Test the voltage between PV+ to PE (Inverter Grounding point or GND point) 

Step3: Test the voltage between PV- to PE (Inverter Grounding point or GND point) 

Step4: Test the voltage between PV+ to PV-

if Vpv+to PE + Vpv-to PE =Vpv+ to pv-,  there is some point short circuit.

if  Vpv+to PE + < 70 and Vpv-to PE < 70 might be normal for the system.

if Vpv+to PE + is more than 200V, the maximum value is equal to Vpv+ to pv- , there might be some point have leakage current.