I have two substances potassium sulfate and potassium carbonate. They are unlabelled and I need to know which one is which. If I add HCl to each substance ie

K2SO4(s) + 2HCl(aq)-->2KCl(aq) + H2SO4(aq)

K2CO3(s) + 2HCl(aq)--> 2KCl(aq) + H2O(l) + CO2(g)

What would I observe in each reaction?

Note that one of the prodcuts of one reaction is a gas. That should be a clue as to what the answer is.

I think I would observe bubbling/fizzing in the second equation, is that correct?

Agreed