Chlorine is a toxic, pungent gas, so tests are done in small amounts in a fume cupboard. The test for chlorine uses damp litmus paper. The paper must be damp so that the gas can dissolve in the water on it.
When damp litmus paper is put into chlorine gas, it does not simply change colour. The litmus paper is bleached and turns white. Chlorine is a bleaching agent, and this result is how you recognise it.
Do not mix this up with the other gas tests. A positive test for chlorine involves damp litmus paper turning white, whereas hydrogen gives a pop and oxygen relights a glowing splint.