A CDI box can be tested using a device called an ohm meter, which measures electrical resistance. The ohm meter can be attached to either input or export wires, ensuring that the CDI box is both receiving power and transmitting it.
Begin by checking the owner's manual for the CDI box to find the device's electrical resistance specs. If the CDI box is in good working order, the readings on the ohm meter should match these specs. Beginning with the input wire, plug the ohm meter in, and turn over the engine to which the CDI box is attached. Make a note of the readings. If they are within the range listed in the manual, proceed to check the output wire.