There are a couple of tests for hydrogen gas, but the most common and reliable one is called the "pop test".

Here's how it works:

1. Collect the gas: Collect the gas you suspect is hydrogen in a test tube or small container.

2. Hold the container near a flame: Carefully hold the container near a lit match or Bunsen burner flame.

3. Listen for the "pop": If the gas is hydrogen, it will react with the oxygen in the air, producing a small explosion and a characteristic "popping" sound.

Caution: This test should only be performed by someone experienced with handling flammable gases and in a safe environment.

There are other tests for hydrogen gas, such as:

* Reaction with copper oxide: When hydrogen gas is passed over heated copper oxide, the oxide is reduced to copper, and water is formed.

* Reaction with chlorine: Hydrogen gas reacts with chlorine gas to form hydrogen chloride gas.

However, the pop test is the most widely used and easily recognizable test for hydrogen.