Sulfur dioxide

Pass the gas over a piece of filter paper soaked in acidified Na2Cr2O7. The paper goes from an orange colour for the Cr6+ to a green colour for Cr3+

Water

Cobalt chloride paper turns from blue to pink in the presence of water.

Acid

Blue litmus paper turns red in the presence of an acid.

OR

Testing using universal indicator shows a pH of less than 7

Carbon dioxide

Passing Carbon dioxide through limewater (calcium hydroxide solution) turns the clear solution milky.

Oxygen

Oxygen gas will relight a glowing splint

Hydrogen

A test tube of hydrogen gas gives off a squeaky pop when lit (do not try to test large volumes of hydrogen gas!)