  We point out the existence of a consistency relation involving the 3-point
function of scalar perturbations which is valid in any inflationary model,
independently of the inflaton Lagrangian under the assumption that the inflaton
is the only dynamical field. The 3-point function in the limit in which one of
the momenta is much smaller than the other two is fixed in terms of the power
spectrum and its tilt. This relation, although very hard to verify
experimentally, could be easily proved wrong by forecoming data, thus ruling
out any scenario with a single dynamical field in a model independent way.
