  We present a novel experimental technique that can differentiate
unequivocally between chaotic light and coherent light with amplitude
fluctuations, and thus permits to characterize unambiguously the output of a
laser. This technique consists of measuring the second-order intensity
cross-correlation at the outputs of an unbalanced Michelson interferometer. It
is applied to a chaotic light source and to the output of a semiconductor
nanolaser whose "standard" intensity correlation function above-threshold
displays values compatible with a mixture of coherent and chaotic light. Our
experimental results demonstrate that the output of such lasers is not
partially chaotic but is indeed a coherent state with amplitude fluctuations.
