  Metallic nanocavity lasers provide important technological advancement
towards even smaller integrable light sources. They give access to widely
unexplored lasing physics in which the distinction between different
operational regimes, like those of thermal or a coherent light emission,
becomes increasingly challenging upon approaching a device with a near-perfect
spontaneous-emission coupling factor $\beta$. In fact, quantum-optical studies
have to be employed to reveal a transition to coherent emission in the
intensity fluctuation behavior of nanolasers when the input-output
characteristic appears thresholdless for $\beta = 1$ nanolasers. Here, we
identify a new indicator for lasing operation in high-$\beta$ lasers by showing
that stimulated emission can give rise to a lineshape anomaly manifesting as a
transition from a Lorentzian to a Gaussian component in the emission linewidth
that dominates the spectrum above the lasing threshold.
