# How to know if a salt is poorly soluble in water or it dissociates completely?

In your inorganic/analytical text you will find a list of ${K}_{\text{sp}}$ values where $\text{sp}$ stands for $\text{solubility product}$. These values MUST be experimentally determined. There are standard rules of aqueous solubility: all nitrates are soluble; all halides are soluble except for those of $P {b}^{2 +}$, $A {g}^{+}$, and $H {g}_{2}^{2 +}$.........etc.
See here for an old answer, and here is another. The questions show how solubility data, ${K}_{\text{sp}}$ values, are used for calculation.