  We propose a realistic scheme for measuring the micromaser linewidth by
monitoring the phase diffusion dynamics of the cavity field. Our strategy
consists in exciting an initial coherent state with the same photon number
distribution as the micromaser steady-state field, singling out a purely
diffusive process in the system dynamics. After the injection of a
counter-field, measurements of the population statistics of a probe atom allow
us to derive the micromaser linewidth. Our proposal aims at solving a classic
and relevant decoherence problem in cavity quantum electrodynamics, allowing to
establish experimentally the distinctive features appearing in the micromaser
spectrum due to the discreteness of the electromagnetic field.
