  Cosmologists are exploring two possible sets of explanations for the
remarkable observation of cosmic acceleration: dark energy fills space or
general relativity fails on cosmological scales. We define a null test
parameter $\epsilon(k,a) \equiv\Omega_m^{- \gamma} d \ln D / d \ln a - 1$,
where $a$ is the scale factor, $D$ is the growth rate of structure,
$\Omega_m(a)$ is the matter density parameter, and $\gamma$ is a simple
function of redshift.
  We show that it can be expressed entirely in terms of the bias factor,
$b(a)$, (measured from cross-correlations with CMB lensing) and the amplitude
of redshift space distortions, $\beta(k,a)$. Measurements of the CMB power
spectrum determine $\Omega_{m 0} H_0^2$. If dark energy within GR is the
solution to the cosmic acceleration problem, then the logarithmic growth rate
of structure $d \ln D / d \ln a = \Omega_m^\gamma$. Thus, $\epsilon(k,a) =0$ on
linear scales to better than 1%. We show that in the class of Modified Gravity
models known as $f(R)$, the growth rate has a different dependence on scale and
redshift.
  By combining measurements of the amplitude of $\beta$ and of the bias, $b$,
redshift surveys will be able to determine the logarithmic growth rate as a
function of scale and redshift. We estimate the predicted sensitivity of the
proposed SDSS III (BOSS) survey and the proposed ADEPT mission and find that
they will test structure growth in General Relativity to the percent level.
