What test can be used to …

Bromine water test can be used to differentiate between saturated and unsaturated hydrocarbons. The addition of bromine (Br₂) gives addition reactions with unsaturated compounds (like alkenes and alkynes). 
When bromine water is added to an unsaturated compound, then bromine gets added to the unsaturated compound and the red-brown colour of bromine water is discharged. So, if an organic compound decolourises bromine water, then it will be an unsaturated hydrocarbon (containing a double bond or a triple bond), but saturated hydrocarbon (alkenes) do not decolourise bromine water.