Answer:
By Ohm's law, resistance is voltage divided by current, so you can determine the resistance of a voltmeter by measuring the total current required to drive it to full scale on each range.
In typical digital voltmeters, the resistance is fixed at 11 or 20 megohms by a resistor divider. This is not often affected by range, because the op-amp that picks up the divided signal contributes negligible resistance to the divider.
In typical analog voltmeters, the resistance is a function of the resistance selected by the range that is placed in series with the meter movement. An example, for a 50 microampere movement is typically 20,000 ohms per volt, so you simply multiply the selected full scale range by 20,000 to get the resistance.