There is an excellent article here on YT that gives a detailed step by step instruction for doing a 12V conversion. It also lists the following test procedure:
"With a small DC test light, hook the light ground clip to the "-" side of the battery. Turn the switch on and observe the voltmeter mounted in the dash - It should rise to 12 volts (or close to it).
a. Touch the battery side of the solenoid with the test light - the light should come on.
b. Touch the "I" post on the front of the solenoid with the test light - the light should come on.
c. Touch the "S" post on the front of the solenoid with the test light - the light should come on.
d. Touch the "+" post on the coil with the test light - the light should come on.
2. If all the above check out, hit the starter button. It should spin over MUCH faster than with the 6 volt system, and will probably crank almost immediately. (Be aware that now you are sending 12 volts to a starter designed for 6 volts. The starter could be damaged by excessive cranking, so if she doesn’t start pretty quickly, let the starter rest between attempts. I personally won’t let the starter run over about 5 to 7 seconds at a pop.) The voltmeter should read about 13.5 to 14.5 volts with the engine running. If it stays at 12, rev up the engine. Mine took this to begin charging. Don't forget to put the air cleaner back on."
The complete article found here: