What is the exact meaning of the term 'isostructural' molecules?
Two molecules are isostructural if they have a similar number of atoms arranged in a similar structure (spatial coordinates). In your example, both $\ce{NO3^-}$ and $\ce{SO3}$ have same number of atoms, and they have similar spatial arrangement as well. So they are isostructural. Sometimes using hybridization of the central atom works in finding out, but not always. For example $\ce{SCl3}$ and $\ce{BF3}$ are isostructural, even though the former is $\text{sp}^3\text{d}$, while the latter is $\text{sp}^2$.