Something has a polar bond when there is a charge difference, for the molecule to be polar there must be an imbalance across the whole molecule (a lack of symmetry) . A few examples;
O=O is not polar because the bonds are non-polar (same electronegativities) and the molecule is linear
CO is polar be because there is a significant difference in the electronegativities of C and O, since there are only two atoms CO must be polar
O=C=O has polar bonds, but is not polar itself because of the symmetry (the polarity is equal but opposite in direction)
A few more examples;
H3C-CH2-CH3 (propane) has two different atoms types but have very similar electronegativities so reamins largley non-polar
NH3 has 3 polar N-H bonds and due to its shape - trigonal pyramidal - a lone pair of electrons occupies one of the corners of the tetrahedral so we have polar N-H bonds pointing in one direction and the lone pair pointing in the opposite direction - charge imbalance
H2O has two polar O-H bonds, its shape (bent) means that these dipoles don't cancel each other out in the fashion of CO2 and so water is polar
Can you tell if these are polar?
SO3 (easy)
propanone (medium)
O3 (hard)