  Let $(M,g)$ be a Riemannian manifold with a distinguished point $O$ and
assume that the geodesic distance $d$ from $O$ is an isoparametric function.
Let $\Omega\subset M$ be a bounded domain, with $O \in \Omega$, and consider
the problem $\Delta_p u = -1$ in $\Omega$ with $u=0$ on $\partial \Omega$,
where $\Delta_p$ is the $p$-Laplacian of $g$. We prove that if the normal
derivative $\partial_{\nu}u$ of $u$ along the boundary of $\Omega$ is a
function of $d$ satisfying suitable conditions, then $\Omega$ must be a
geodesic ball. In particular, our result applies to open balls of
$\mathbb{R}^n$ equipped with a rotationally symmetric metric of the form
$g=dt^2+\rho^2(t)\,g_S$, where $g_S$ is the standard metric of the sphere.
