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Overall molecular polarity is determined by the net dipole, which is equal to the sum of all the individual bond dipoles (i.e. the electronegativity differences between two bonded atoms). For example, if an atom has a much stronger bond dipole towards one side, then that molecule has a net dipole and thus, is polar. Some molecules have bond dipoles that may end up cancelling out due to symmetry.
A good way to think about this is to compare a known polar molecule to a known nonpolar molecule. Take H2O and CH4 for example. We know water is polar, and we can see that the net dipole moments do not cancel, and the molecule is left with a positive dipole by the hydrogen atoms and a negative dipole by the oxygen atom. In contrast, methane is nonpolar and although the bonds are not perfectly covalent, the dipole moments are all equal and cancel out. This means there is no net positive/negative charge anywhere, and the molecule is nonpolar overall.
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