  For every involution $\mathbf{w}$ of the symmetric group $S_n$ we establish,
in terms ofa special canonical quotient of the dominant Verma module associated
with $\mathbf{w}$, an effective criterion, which allows us to verify whether
the universal enveloping algebra $U(\mathfrak{sl}_n)$ surjects onto the space
of all ad-finite linear transformations of the simple highest weight module
$L(\mathbf{w})$. An easy sufficient condition derived from this criterion
admits a straightforward computational check for example using a computer. All
this is applied to get some old and many new results, which answer the
classical question of Kostant in special cases, in particular we give a
complete answer for simple highest weight modules in the regular block of
$\mathfrak{sl}_n$, $n\leq 5$.
