Question

how can we test carbon dioxide gas

Answer 1

We can bring a flame near the test tube and if the flame extinguishes, then the gas is carbon dioxide.  CO2 extinguishes a flame.

Answer 2

A lighted wooden splint goes out in a test tube of carbon dioxide but this happens with other gases, too. It is better to bubble the test gas through limewater - calcium hydroxide solution. Carbon dioxide turns limewater cloudy white.