Alinear molecule composed of two atoms is polar when

there is a electronegativity difference between the two atoms.

Explanation:

A polar covalent bond is defined as the bond which is formed when there is a difference of electronegativities between the atoms. It is also defined as the bond which is formed due to the unequal sharing of electrons between the atoms.  An electronegative atom is the one which attracts the shared pair of electron towards itself.  Example: is polar as florine is more electronegative than hydrogen and thus bears a partial negative charge and hydrogen bears a partial positive charge.

Non-polar covalent bond is defined as the bond which is formed when there is no difference of electronegativities between the atoms. It is also defined as the bond which is formed due to the equal sharing of electrons between the atoms.  Example: