Identifying covalent character in ionic compounds when varying multiple ions
You will have to consider two factors here: polarising power of the cation and polarisability of the anion. 
There is more than one part of Fajan’s rule(Source-J. D. Lee, Concise Inorganic Chemistry):


  
  A small positive ion favours covalency. 
  A large negative ion favours covalency. 
  Large charges on either ion, or both ions, favours covalency. 
  Polarisation and hence covalency is favoured if the positive ion does not have noble has configuration.
  


According to these, the $\ce{Li+}$ ion is more polarising than $\ce{Na+}$ ion, and $\ce{Cl-}$ ion is more polarisable than $\ce{F-}$ ion. 

Since both factors support it, $\ce{LiCl}$ is more covalent than $\ce{NaF}$. 

Somtimes, like in your question, both the factors support each other. Sometimes, when they are opposite, like in the case of $\ce{LiF}$ and $\ce{NaCl}$, it is tough to determine relative covalency using Fajan’s rule.