A catalyst is a substance that speeds up a chemical reaction. It is not used up in the reaction, so it is not a reactant and it does not appear in the overall equation. At the end of the reaction it is still there, chemically unchanged, and its mass is the same as at the start.
This gives two clues for identifying a catalyst. First, the reaction is faster when the substance is added. Second, the substance can be filtered off, dried and weighed, and it has the same mass as before.
Hydrogen peroxide breaks down very slowly into water and oxygen gas. When a black powder such as manganese dioxide is added, the reaction is much faster and the mixture fizzes. For example, a reaction that gives 4 cm3 of gas in a minute without the powder and 40 cm3 with it is 10 times faster. A glowing splint held in the gas relights, which is the test for oxygen. You can compare different black powders by measuring the volume of oxygen collected in the same time. Copper powder is another example. It makes the reaction of zinc with sulfuric acid produce hydrogen faster, and the copper is still there at the end.