Covalent bonds within molecules (inter) are strong. If some of these bonds are polar the molecule can have a permanent dipole. This influences the size of the Van der Waals forces between molecules (intra).
Van der Waals forces and permanent dipoles.
The steps involved in deciding whether a molecule has a permanent dipole include:
Deciding if there is a significant electronegativity difference between atoms in the covalent bonds. Are there polar bonds in the molecule?
From the Lewis diagram predict the shape of the molecule.
Is the shape symetrical with a zero vector addition of dipoles (non polar molecule)
Is the shape asymetrical with a non zero vector addition of dipoles ( polar molecule)