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If you have a voltmeter and DC power pack, connect jumper wires from the power pack's variable DC terminals to the headlight wiring. Turn the throttle on the power pack up very gradually until the headlight illuminates at the level it has been illuminating on DC at full throttle, and measure the voltage.
If it's 12 volts or slightly over, you don't need a resistor; however, I think some decoders recommend a low-value resistor to prevent a current surge through a cold bulb filament from burning out the decoder. Check the decoder documentation and see what they recommend for the headlight wiring.