Determine polarity of covalent bond with formal charge
All bonds between two atoms of different elements are polar to some extent. To determine how polar, you need to check the elements’ electronegativities and only them.

Formal charges are exactly what their name implies: a formality. When drawing Lewis structures you should attempt to generate all-octet structures but often this isn’t possible without moving electrons entirely between atoms. In that case, formal charges are assigned to rationalise e.g. a nitrogen with only four electrons.



Check out these structures of nitric acid, sulphur trioxide and carbon monoxide. (Resonance structures applicable to $\ce{HNO3}$ and $\ce{SO3}$ not displayed.)  All bonds are polarised towards the oxygen, since oxygen’s electronegativity is $3.5$; the second highest amoung the non-noble-gas elements. Note especially that the bonds in $\ce{CO}$ are polarised towards oxygen despite oxygen bearing a formal positive charge. (There is a lot more to $\ce{CO}$ which is shown well in Martin’s answer including the molecular orbitals)