Question

how can we identify the presence of carbon dioxide and hydrogen gas

Answer 1

if we bring calcium oxide near the co2 gas it would thus milky in colour thus helping us to identify co2
as far as h2 is concerned it burns with a pop sound the sound would indicate the presence of h2

Answer 2

Impure Hydrogen burns with a pop sound in air and pure Hydrogen burns with a pale blue flame.

Carbon dioxide turns lime water milky and has no effect on KMnO4.