Polar or Nonpolar?

A polar bond is a covalent bond between two atoms where the electrons forming the bond are unequally distributed. This causes the molecule to have a slight electrical dipole moment where one end is slightly positive and the other is slightly negative. The charge of the electric dipoles is less than a full unit charge, so they are considered partial charges and denoted by delta plus (δ+) and delta minus (δ-). Because positive and negative charges are separated in the bond, molecules with polar covalent bonds interact with dipoles in other molecules. This produces dipole-dipole intermolecular forces between the molecules. You can state if a covalent bond is polar or not by looking at the difference of electronegativity of its atoms. For example, the difference of electronegativity of the atoms H and O is significant (greater than 0.4); the molecule $H_{2}O$ is polar. Now consider $CH_{4}$ . The difference of electronegativity of the two atoms C and H is NOT significant (less than 0.4); this molecule is, in fact, nonpolar. So, in order to state whether a molecule is polar or not, is this method always valid?