  We study a finite volume scheme approximating a parabolic-elliptic
Keller-Segel system with power law diffusion with exponent $\gamma \in [1,3]$
and periodic boundary conditions. We derive conditional a posteriori bounds for
the error measured in the $L^\infty(0,T;H^1(\Omega))$ norm for the
chemoattractant and by a quasi-norm-like quantity for the density. These
results are based on stability estimates and suitable conforming
reconstructions of the numerical solution. We perform numerical experiments
showing that our error bounds are linear in mesh width and elucidating the
behaviour of the error estimator under changes of $\gamma$.
