  Single scalar field inflation with a generic, non-quadratic in derivatives,
field Lagrangian is considered. It is shown that non-Gaussianity of curvature
perturbations is characterized by two dimensionless amplitudes. One of these
amplitudes can be expressed in terms of the usual inflationary observables --
the scalar power, the tensor power, and the tensor index. This consistency
relation provides an observational test for the single scalar inflation.
