Question

how would you confirm that a colourless liquid given to you is pure water​

Answer 1

drink it

or just check the boiling point

or water can also be detected using blue anhydrous cobalt(II) chloride. This turns pink in the presence of water.

Answer 2

By finding the boiling point of a given colourless liquid. If the liquid boils at 100°C at atmospheric pressure, then it is pure water. This is because pure substances have fixed melting and boiling point.