How to determine if a liquid is water?
We can use the physical properties of water such as density, melting point and boiling point, etc to determine if a liquid is water. Water has a density of 1 gm/cm^3 or 1000 kg/m^3. We can measure the mass of one cm^3 or 1 m^3 of the unknown liquid and check against the mass of equivalent volume of water. Water freezes at 0 degrees Celsius and converts back to liquid state, when the temperature increases from 0 degree Celsius. Water boils at 100 degrees Celsius and converts to gas phase (water vapors). If the unknown liquid freezes at 0 degrees Celsius and boils at 100 degrees Celsius and converts to gas phase, then we can check it for further similarities with water (such as ability to extinguish fire, etc.).
Hope this helps.