Hydrogen, oxygen and carbon dioxide can be identified using different tests.

Hydrogen

Putting the flame of a lighted wooden splint in a test tube of hydrogen makes a ‘squeaky pop’ sound.

Oxygen

Putting a glowing wooden splint in a test tube of oxygen, causing the splint to relight.

Carbon dioxide

Bubbling carbon dioxide through limewater changes it from colourless to ‘milky’ due to the formation of a white precipitate of calcium carbonate.