How to determine sulfate ions in a solution by precipitation?
Acquaint yourself with the so-called solubility rules. Depending on which cation sulfate is paired with, the sulfate compound may or may not be soluble. Generally, compounds containing sulfate ($\ce{SO_4^2-}$) compounds are soluble. (For essentially the same reason that sulfuric acid $\ce{H2SO4}$ is a strong acid, if you're interested.)

Well-known insoluble sulfate compounds are $\ce{PbSO4, SrSO4, CaSO4}$, and $\ce{BaSO4}$. Note that all of these cations have a somewhat high (+2) charge and are large. 

If sulfate anions are present in your aqueous solution and you a a soluble compound containing $\ce{Pb^2+, Sr^2+}$, etc, then those cations will form an insoluble precipitate with the sulfate.