Consider the following reaction,
Ca(OH)2 + CO2 → CaCO3 + H2O
(non metallic oxide)
Here, calcium hydroxide (Ca(OH)2) is a base, and it reacts with carbon dioxide (CO2) to give salt (calcium carbonate) and water. Comparing the above reaction with,
Base + Acid → Salt + Water
One can see that CO2 acts as an acid. Hence, one can prove that non-metallic oxides are acidic in nature.