Catalase is used to break up hydrogen peroxide into water and oxygen. I can demonstrate this on a potato by cutting it up and dripping the substance on it, but how do I prove that the bubbling gas released is oxygen? I can tell from the chemical equation, but the question needs me to demonstrate something practically. Is it possible to prove that it is oxygen by dipping a piece of pure iron into the foam and see if it rusts?

do it in a sealed vessel and use a gas syringe to measure the increase of volume.

Or see if flames grow larger when exposed to the gas.

I have leaned in microbio that a dye called reazurin can be used to detect oxygen. It turns pink in the presence of oxygen.

If you are using dialysis tubing a little research into what can permeate through it might help. I don't want to give too much away but look at an 02 molecule;)

just blow the collected gas over a lit cigarette, if it catches fire, it`s O2

The test we have always learned in chem is to see if it will light a glowing splint.