It has something to do with the electron configuration. Like H-S-H, it is not linear. And the center of H is not the same as S. So you can analysize the electron configuration.Therefore you can determine whether it is polar or not.
linuxux, you may wish to consider the relative electronegativity values of each atom that makes up the molecule.
As an aside, not all molecular gasses are polar; take hydrogen, chlorine, methane for example. Polarity depends not only on the electronegativity of each atom, but also the actual shape of the molecules themselves.
i already did all the lewis dot diagrams, they all fit into the electrostatic repulsion geometry, the only one that didn't have an all symmetric geometry was [tex]PCl_{3}[/tex], unless i did something wrong, i haven't considered electro negativity yet however.