To compare the electrostatic forces between the molecules of different substances, the most effective procedure would be:
Heat each substance in solid form to its melting point. Measure the quantity of each substance that has melted.
This approach allows you to directly assess how much energy is required to overcome the electrostatic forces holding the molecules together in their solid state. The amount of substance that melts at a given temperature reflects the strength of the intermolecular forces, as substances with stronger electrostatic forces will require more energy (and hence a higher melting point) to change from solid to liquid.