  The electromagnetic radiation emitted by an ultra-relativistic accelerated
electron is extremely sensitive to the precise shape of the field driving the
electron. We show that the angular distribution of the photons emitted by an
electron via multiphoton Compton scattering off an intense
($I>10^{20}\;\text{W/cm$^2$}$), few-cycle laser pulse provides a direct way of
determining the carrier-envelope phase of the driving laser field. Our
calculations take into account exactly the laser field, include relativistic
and quantum effects and are in principle applicable to presently available and
future foreseen ultra-strong laser facilities.
