Question

what is the best way of identification a given colourless liquid to be water or not

Answer 1

to find a colorless liquid that it is water or not 
it will be odourless 
it will be absorbing every color
the alkalies and salts will be dissolving in it

Answer 2

Water can be identified by a simple experiment.

If we dissolve copper sulfate crystals in water,the colour of water will be turned to blue solution.Now if we strongly evaporate the blue solution,we will get white coloured copper sulfate crystals.we all know the colour of anhydrous copper sulfate is nothing but white. In this way we can easily identify a colourless liquid tobe water or not.