  If general relativity is the correct theory of physics on large scales, then
there is a differential equation that relates the Hubble expansion function,
inferred from measurements of angular diameter distance and luminosity
distance, to the growth rate of large scale structure. For a dark energy fluid
without couplings or an unusual sound speed, deviations from this consistency
relationship could be the signature of modified gravity on cosmological scales.
We propose a procedure based on this consistency relation in order to
distinguish between some dark energy models and modified gravity models. The
procedure uses different combinations of cosmological observations and is able
to find inconsistencies when present. As an example, we apply the procedure to
a universe described by a recently proposed 5-dimensional modified gravity
model. We show that this leads to an inconsistency within the dark energy
parameter space detectable by future experiments.
