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+
Cobalt chloride paper turns from blue to pink in the presence of water.
Blue litmus paper turns red in the presence of an acid.
Testing using universal indicator shows a pH of less than 7
Passing Carbon dioxide through limewater (calcium hydroxide solution) turns the clear solution milky.
Oxygen gas will relight a glowing splint
A test tube of hydrogen gas gives off a squeaky pop when lit (do not try to test large volumes of hydrogen gas!)