how to know clarify if the subtance is polar or nonpolar?

Answer 1

Two things must be true to have a polar substance.
a. There must be a difference in polarity between the central atom and the other atoms (or for a diatomic material there must be a difference between the two atoms) AND
b. The molecule must NOT be symmetrical (in a three dimensional sense).
Unless BOTH a and b are true, the molecule will not be polar.
For example, CF4 is not polar(a is true but CF4 is symmetrical).
HF is polar, CO2 is not polar. CHF3 is polar.