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Look up the electronegativity of each atom. Subtract the smaller number from the larger. This gives you the difference. For H2, Cl2, N2, O2, the answer is simple since there is no electronegativity difference. These are all covalently bonded. If electronegativity difference is between 0.3 and 1.7 then, generally, the bond is polar. Above 1.7 is generally considered ionic. Just because the bonds are polar, however, doesn't mean the molecule is. methane and carbon dioxide, for instance are non-polar because the bond dipoles are balanced and cancel one another.