How to test the gases evolved from this decomposition?

When you heat Lead(II) nitrate it decomposes to solid lead(II) oxide plus gases (Nitrogen dioxide NO₂ and Oxygen O₂). One can test the gases by standard chemical tests: for O₂ use a glowing splint it will re-ignite in oxygen. For NO₂ (a reddish brown gas) you can observe its brown fumes; you might bubble the gas through potassium hydroxide solution and observe formation of nitrate and nitrite ions, or measure its toxicity/colour. 

When the white or colourless crystals turn into a yellow-solid and brown fumes appear, you know decomposition has occurred. 

Equation: 2 Pb(NO₃)₂ (s) → 2 PbO (s) + 4 NO₂ (g) + O₂ (g)