How to find minimum covalent character also tricks if any.?

According to Fajan, if two oppositely charged ions are brought together, the nature of the bond between them depends upon the effect of one ion on the other.

When two oppositely charged ions (say A+ and B- ) approach each other the positive ion attracts electrons on the outermost shell of the anion and repels its positively charged nucleus. This results in the distortion,deformation or polarization of the anion. If the polarization is quite small, an ionic bond is formed,  while if the degree of polarization is large, a covalent bond results.

Thus the power of an ion (cation) to distort the other ion is known as its polarization power and the tendency of the ion(anion) to get polarized by the other ion is known as its polarisability.  Greater the polarization power or polarisability of an ion, greater will be its tendency to form a covalent bond.