[jdurg]

NO₂ also has that "stuck in traffic behind an old beat up car with no catalytic converter" smell to it.  Kind of smells like bleach, but it distinctly different.

[bramgo]

Oooo cool ! But isn't that cheating !? I think  jgaoxx06 wanted to know about a specific chemical test for NO₂ and oxides of sulfur. My teachers wouldn't have been too impressed if I told them that a test for these gases was to test them with specific kits which test for the gas in question!

- You are right, it is a bit cheating maybe. Allthough it was my prof who told me to do it like that.

- However I heard there's a more impressive way of doing it:
When reactions take place there are photons exchanged (Chemoluminescence). There is a cool theoretical function to know how much light is emitted. (People familiar with electronics will probably know something more about this. I think it's also related to the working of Light Emitting Diodes. I recall something like that from my earlier studies.)
In short... You could let NO react with ozon (O₃) for instance. It will emit light. This can be measured with a photodetector.