CdS is a solid and it should not dissolve because it is a precipitate. If you want to determine the concentration of Cd in a precipitate, you can convert the CdS to CdSO4 by oxidation. You can also convert CdS to Cd-nitrate which is completely soluble in water by react the CdS with hydrogen peroxide then Nitric Acid.
Why do you suspect/think that CdS might be a liquid? According to my 1960's chemistry book, the solubility product of CdS is around 10 to the power - 26, so a very water insoluble substance. As stated in the above answer, conversion into another salt is the only option to obtain a soluble form of Cd.