Which are non-polar molecules? 

There are certain molecules that have zero dipole moment and some having non-zero dipole moment. Molecules that have zero dipole molecule considered as non-polar molecules.
There are two major conditions in which molecule has zero dipole moment:
1. There is negligible or zero electronegativity difference between the bonded atom.
2. Molecule has symmetrical shape due to which the resultant dipole moment becomes zero.

XeF₄-square planar-symmetrical shape

BF₃-trigonal planar-symmetrical shape

SiF₄-tetrahedral-symmterical shape