When you dim the light, the dimmer circuit reduces the effective voltage. Any analog voltmeter can be reasonably easily adapted to show the voltage change. It would be best to start with a voltmeter that can display the normal voltage without dimming the light. Then it will show a reasonable approximation of the voltage when the light is dimmed without any modification to the meter. If you have a meter designed for a lower voltage, it can be adapted using a series resistor. If it is designed for a higher voltage, you could use it without modification, but the reading would be a lower point on the scale. If the meter is designed for a much higher voltage, you might want to modify it or possibility use a small transformer. For assistance with that you would need to show details of the meter.
The above assumes starting with an AC voltmeter. If you have a DC meter, refer to the answer by crj11 for converting it to read DC.