  If the FRW metric is a good approximation on large scales, then the distance
and the expansion rate, as well different notions of distance, satisfy certain
consistency conditions. We fit the JLA SNIa distance data to determine the
expected amplitude of the violation of these conditions if accelerated
expansion is due to backreaction. Adding cosmic clock and BAO expansion rate
data, we also model-independently determine the current observational limits on
such violation.
  We find that the predicted maximum backreaction amplitude $|k_H|\lesssim1$
(95% C.I.) is of the same order as the current observational constraints
$|k_H|\lesssim1$, the precise numbers depending on the adopted fitting method
(polynomials or splines) and stellar population evolution model. We also find
that constraints on the value of $H_0$ determined from expansion rate data are
sensitive to the stellar evolution model. We forecast constraints from
projected LSST+Euclid-like SNIa plus Euclid galaxy differential age data. We
find improvement by factor of 6 for the backreaction case and 3 for the
model-independent case, probing an interesting region of possible signatures.
