  We propose a few tests of Seiberg-Witten solutions of $\mathcal{N}=2$
supersymmetric gauge theories by the instanton calculus in twisted gauge
theories. We re-examine the low-energy effective abelian theory in the presence
of sources and present the formalism which makes duality transformations
transparent and easily fixes all the contact terms in a broad class of
theories. We also discuss ADHM integration and its relevance to the stated
problems.
