Question

How we will get to know which compound is more stable

Answer 1

Depends on what perspective you are asking this from.

If you are taking an introductory class and you want a good qualitative guide to assessing which resonance forms are most important and contribute the most to the hybrid, follow these general principles (roughly stated in order of importance). Note that these rules are best applied to resonance forms lacking unpaired electrons.
1. Minimization of point charges. The fewer the point charges, the greater the contribution of the resonance structure will be.
2. Full octets. All else being equal a resonance form where each atom possesses a full octet of electrons will make a greater contribution to the hybrid.
3. Stabilized negative charges. If you absolutely must have a negative charge in your resonance structure, it should be on the atom best able to stabilize it. Here, follow the general rules for basicity. Across the periodic table, greater electronegativity = greater stability, so O- > N- > C- . Down the periodic table, greater size = greater stability, so S- > O- etc.
4. Stabilize positive charges. If you absolutely must have a carbon with less than a full octet, it's best to put it on the most substituted carbon possible (tertiary > secondary > primary).
5. (special case) Aromaticity. If any of your resonance forms are aromatic, they will have a disproportionate contribution to the resonance hybrid. The classic example is tropone, which has significant negative charge buildup on the oxygen because this resonance form is aromatic.