The purity of water can be tested by evaporating it (to dryness) on an evaporating dish or by measuring its boiling point.
Pure substances have precise melting and boiling points. Pure water will melt at 0°C and boil at 100°C. The water vapour will also condense at 100°C. If impurities are present:
The greater the amount of dissolved substances present, the more the melting point or boiling point changes from the temperature expected for pure water.