Heating metal salts in a dry test tube and observing the change taking place there, is a preliminary way of detecting the presence of certain radicals (ions) in it. While most metal oxides decompose upon heating, to produce metal oxide and brown coloured nitrogen dioxide and oxygen, sodium nitrate (and potassium nitrate) upon heatling decomposes to produce sodium nitrite and colourless and odourless oxygen gas. The colourless oxygen gas is characterized when it rekindles a glowing wooden splinter. The nitrite of sodium (and potassium, of course) is stable to heat.
2NaNO3 (s) → 2NaNO2 (s) + O2 (g)