How to know covalent bond with highest polarity?
There are quite a few ways of quantifying bond polarity; many rely on differences in electronegativity, elemental ionization energies, and measurement of bond dipole moment. Electronegativity is possibly the most useful.
Electronegativity is conceived (what do I mean by this?) to be the ability of an atom in a molecule to polarize electron density towards itself. There are simple and complicated tables of electronegativity, of which the Pauling Scale is the earliest and is still useful. On this scale, the alkali metal Fr had a value of 0.7, and fluorine, the most electronegative element, had a value of 4.0 (the units are quite arbitrary).
The difference in electronegativity (
To return to your question (finally!), the most polar covalent bond is likely between
Please note that the above treatment lies well beyond A-level standard.