Apply 12 volts to the contacts on the back of the bulbs;if they go POOF!!,they're 6 volt. Sorry,just kiddin'. I have had cases where the writing on the back of the bulbs was gone,I just connected them to a 6 volt power source (battery charger or battery);if they were excessively dim or orange,they were 12 volt,if they were average brightness they were 6 volt.
Speed
