COCl2 is a polar molecule because the dipole between the carbon and the chlorine atoms is not equal to the dipole between the carbon and oxygen atoms. Molecules are only non-polar if they contain no polar bonds or if the partial charges cancel out each other.
To determine if a molecule is polar, it is important to first analyze all the bonds. In COCl2 all three of the bonds have dipoles because the electronegativity of oxygen, chlorine and carbon are all different. The direction and magnitude of these dipoles can be determined by calculating the difference in elecronegativity. If there are polar bonds (bonds with partial charges), and those bonds do not completely cancel each other out, the molecule is polar.