  We present a simple and flexible technique for identifying the onset of
coherent emission in lasers, from the mesoscale to the nanoscale, which makes
use of photon counting and a small amplitude modulation added to the pump. The
optimal modulation frequency is obtained from the radiofrequency power spectrum
of the unperturbed laser emission. The identification of the lasing onset rests
on the appearence of a resonance in the response of the zero-order
autocorrelation function ($g^{(2)}(0)$) plotted as a function of the pump rate.
The intrinsic simplicity of this technique and its use of photon counting make
it an excellent tool for certifying the onset of laser emission in nanoscale
sources.
