By looking at the salt how can we identify whether it is a double salt or a complex salt?

we cannot distinguish bewtween double salt and complex salt by simple observation because the difference arise between them when dissolved in solution,In solution double salt give individual property of cation and anion,whereas complex salt does not give.Even though, both double and complex salts contain two metal ions and may be large salts with have many anions attached to them, Also, when double salts are tested for presence of their constituent ions, for example KClMgCl.6H2O when tested for presence of K, will give a positive result, whereas a complex ion would not.