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You can look at the difference in electronegativity to figure out if a molecule is polar or look at its shape. If a molecule is symmetric it is non-polar, i.e. CH4 would be non-polar because it's symmetrical. An example of a polar molecule is NH3, which is not symmetric.
I believe we have to look at the electronegativity between bonds and if the difference is greater than 0.4, then the bond is polar. If the difference is less than 0.4, then the bond is nonpolar. If there are no polar bonds, then the molecule as a whole should be nonpolar.
You have to consider shape and the dipole moments. For example, if a molecule had two polar bonds both pointing towards the central atom and the shape was linear, the dipole moments would cancel (vectors) and the molecule would be non-polar. However, if you had water (where the shape is a tetrahedral) the molecule would be polar because the dipole moments would not cancel.
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