Distinguishing between potassium chloride and sodium chloride
Per Wikipedia:


  ..., MgSO4·H2O, can be combined with a solution of potassium chloride to produce potassium sulfate.


So, cooling an aqueous solution KCl in the presence of Epsom's Salt:

$\ce{2 KCl + MgSO4 &lt;=&gt; MgCl2 + K2SO4}$

creates potassium sulfate, which falls out of the solution forming distinct orthorhombic crystals.

If Na2SO4.7H2O is created instead from aqueous NaCl in place of KCl, you will get something that resembles a gluey paste.