Question

Explain how you could find out whether of not an unknown liquid is water

Answer 1

I know that density of water is 1000kg per cubic meter or 1g per 1ml
I would take beaker and scales.
1) I would measure liquid's weight (in g)
2) measure volume (in mL)
3) divide weight by measure
4) if would get 1 as result - it's water
5) if it's above or below 1 - it's not the water

Answer 2

 use the different characteristics of water such as freezing and boiling points, waters density, or using your five senses. Smell it, but don't taste it, feel it, listen to it. It might crackle or pop if it has carbon.