There are a number of ways to observe the release of a gas. If you perform your experiment underwater, you can see the bubbles that a gas will form. In a sealed container, the release of a gas will increase the pressure in the container (and can do so sufficiently to cause an explosion, in some cases). Some gases (such as nitric oxide) are visible. Many gases have a detectable odor. Chlorine gas can be observed either visually (it is green) or by smell (it causes a choking sensation). Cyanogen can be observed by the people who drop dead when they inhale it. So there are many different methods.
Hydrogen
Carbon Dioxide
evaporation
Oxygen is given off as a by-product os photosynthesis
respiromterrespirometer
When you place a lighted splint into, say, a test tube, where you suspect hydrogen gas is being given off, a 'pop' sound indicates the presence of the gas.
if an experiment is done in a test tube and hydrogen gas is given off if a lit splint is placed inside the test tube there will be a pop.
None, no gas is given off.
Cyanoacrylate gas is given off.
Carbon dioxide is the gas given off. Oil also gives off the same gas when burned.
hydrogen eg:Na+HCl------>NaCl+H2
Carbon Dioxide is given off. (CO2)
Oxygen gas
Hydrogen gas.
Oxygen is given of
Oxygen
Oxygen.