By Fajans' rules, we can easily find out which compound shows covalent character.
Example: Among $\ce{NaCl}$, $\ce{MgCl2}$, $\ce{AlCl3}$ which one is more covalent?
Answer: There is a point in Fajans' rules that the compound which has more charge number that compound will show covalent character. In $\ce{AlCl3}$, Al has the highest oxidation state (+3) so it will show more covalent character than $\ce{NaCl}$ and $\ce{MgCl2}$.
The point is that since all three compounds I have mentioned have chlorine in common, it was easy to catch the solution but if the compounds were given as $\ce{NaF}$ and $\ce{LiCl}$, which one shows more covalent character, and how can we apply Fajans' rules?