Question

How we tell if a compound is soluble ?

Answer 1

solubility rules.

Answer 2

Like dissolves like. A polar molecule is soluble in a polar solution. A non-polar molecule is soluble in a non-polar solution. Usually solubility means "is it soluble in water". What is a polar molecule therefore you need to identify if your compound is polar

Answer 3

@KenLee "like dissolves like" only describes \(general\) tendencies, there are plenty of exceptions to both sides of this rule. In order to really determine whether a substance is soluble in water, we either:

put a small amount of the substance in water and visibly observe, or
use solubility rules (if it's ionic) to determine, or
use KSP values