what can we assume about their anion, if a salt is soluble in water. (salt analysis)

If the salt is soluble in water, the anion is probably NO₃^2- or ClO₄^- .
It can be any anion if the cation is group 1 element except for lithium phosphate.
It can be Cl^- or Br^-  of any cation except for metals like Ag⁺, Pb²⁺