When an acid reacts, it can produce two different gases: hydrogen gas (H2) and carbon dioxide gas (CO2).
To test for hydrogen gas, you can use the "pop" test. Collect the gas in a test tube and carefully bring a lit splint near the mouth of the tube. If the gas is hydrogen, you'll hear a distinctive "pop" sound as the gas ignites.
To test for carbon dioxide gas, you can perform the limewater test. Pass the gas through a solution of limewater (calcium hydroxide). If the gas is carbon dioxide, the limewater will turn cloudy or milky due to the formation of calcium carbonate.
Hope that helps! Let me know if you have any more questions.