How to distinguish between polar and non-polar molecules/compound? for instance, one that you have not studied before but have to know if its polar or non-polar. can you distinguish it from the basic structure of the molecule?

a molecule is polar if we find electronegativity differences between two differing atoms.
when there is large difference between electronegativity values, the more polar the molecule is.
 when  the electrons are shared equally between the nuclei.then the molecule is non polar
  in Cl2 moleculethere is no  no polar bonds because the electron charge is  same on both atoms.
 It is therefore a nonpolar molecule.
bonds in hydrocarbon molecules,  as hexane, C6H14, are polar,