I downvoted because any answer to the question has a bigger problem to address than how to distinguish $\ce{H2SO4}$ and $\ce{H2SO3}$. That is the fact that $\ce{H2SO3}$, for all intents and purposes, does not exist. That's why your Google search doesn't tell you anything; it can't tell you what chemical reaction to use to distinguish something that exists from something that doesn't.
There's nothing wrong with not knowing something, but it can be a problem when the entire premise of a question relies on you practically assuming the existence of $\ce{H2SO3}$ (or it being comparable in some way to $\ce{H2SO4}$). The answer then has to address that issue, instead of focusing on the actual question posed (how to differentiate them).
If you're specifically referring to an aqueous solution of sulfur dioxide, then I suggest editing your question to clarify.
Downvoting sounds cruel, but it is not a judgment of you. It just means that as it stands, in my opinion, the question is not generally a helpful one to other people, including future visitors, for the reasons outlined above. That is not necessarily your fault.
FWIW, I don't necessarily agree with the closure, but I do stand by my vote (for now, at least).