One definition of Lattice Energy is its a type of potential energy stored in the crystal structure of a solid. Thus, the lattice energy is the energy required to break apart an ionic solid and convert its component atoms into gaseous ions.
Most ionic compounds' lattice energies are high. Ions do not easily detach themselves from their crystal structure into a gaseous state. So ionic solids no not go from a solid state to gas state at ordinary temperatures. They can be melted by applying sufficient thermal energy to overcome the crystalline lattice. Therefore, the higher the lattice energy is of an ionic compound, the higher its melting point is.
For NaCL, the melting point is about 1474oF or 801oC, and its lattice energy is about 787 kJ/mol.
For KCL, the melting point is about 1418oF or 770oC, and its lattice energy is about 715 kJ/mol.
Recall that energy needed to break apart an ionic crystal (when it is dissolved) comes comes from the interaction of ions in the crystal solid with the molecules of the solvent. Releasing the lattice energy of an ionic solid is what an ionic solid depends on to dissolve itself in a solvent. Therefore, the lower the lattice energy, the higher the quantity an ionic solid that can be dissolved in any quantity of solvent.
So set up a simple experiment that either measures the melting temperature of each salt, or alternatively (starting with an identical quantity) of solid crystal in your solution, which solution has more solid crystal left after heating them up sufficiently to the lower one's melting temperature.