Q.How is the presence of SO2 detected ?

Step 1 — Physical/sensory detection.

$SO_2$ has a sharp, pungent, suffocating odour that is immediately noticeable even at low concentrations, allowing quick qualitative identification.

Step 2 — Chemical test with acidified dichromate.

$SO_2$ is a reducing agent; passed through filter paper soaked in acidified potassium dichromate solution, it reduces the orange $Cr_2O_7^{2-}$ (Cr in +6) to green $Cr^{3+}$ (Cr in +3):

the colour change orange → green is a definitive test.

Step 3 — Alternative chemical test. …

Combine the simple sensory clue (pungent odour) with the standard chemical confirmatory tests that exploit $SO_2$'s reducing property agains …

• Don't rely on smell alone as a rigorous chemical "detection" answer — the expected chemistry answer also needs the dichromate (or permanganate) colour-change test. …