I do not think Mn substitutes Ba in (all RE)-123 compounds. Its ionic radio is too small when compared with the Ba one which makes such a substitution hard to occur. I suspect that it substitutes Cu instead. For instance, you may check any cationic substitution by looking for tiny changes in the lattice parameters of the unit cell or by quantitative SEM analysis. Other macroscopic properties of the substituted phase may change as well as the temperature dependence of the electrical resistivity or even magnetization.