  We analyze the Jordan-Brans-Dicke model (JBD) of gravity, where deviations
from General Relativity (GR) are described by a scalar field non-minimally
coupled to gravity. The theory is characterized by a constant coupling
parameter, $\omega_{\rm JBD}$; GR is recovered in the limit $\omega_{\rm JBD}
\to \infty$. In such theories, gravity modifications manifest at early times,
so that one cannot rely on the usual approach of looking for inconsistencies in
the expansion history and perturbations growth in order to discriminate between
JBD and GR. However, we show that a similar technique can be successfully
applied to early and late times observables instead. Cosmological parameters
inferred extrapolating early-time observations to the present will match those
recovered from direct late-time observations only if the correct gravity theory
is used. We use the primary CMB, as will be seen by the Planck satellite, as
the early-time observable; and forthcoming and planned Supernov{\ae}, Baryonic
Acoustic Oscillations and Weak Lensing experiments as late-time observables. We
find that detection of values of $\omega_{\rm JBD}$ as large as 500 and 1000 is
within reach of the upcoming (2010) and next-generation (2020) experiments,
respectively.
