Molecules in a gas don't stick together, so they can't form drops as a liquid does. How could you compare two different gases to determine which gas particles are more strongly attracted to each other?

For ideal gas, there is no attraction between its molecules. But for real gas molecules, there are attractive forces in between molecules. Those are called Van der Waals forces.

To determine which gas particles are more strongly attracted to each other, you can compare two different gases at constant pressure but different temperatures. Because of the Van der Waals forces, the increase in volume due to temperature will be smaller than what is predicted by the ideal gas law. The larger the difference from the ideal gas law's prediction, the larger the intermolecular attractive force is.