  New technological developments allow to explore the quantum properties of
very complex systems, bringing the question of whether also macroscopic systems
share such features, within experimental reach. The interest in this question
is increased by the fact that, on the theory side, many suggest that the
quantum superposition principle is not exact, departures from it being the
larger, the more macroscopic the system. Here we propose a novel way to test
the possible violation of the superposition principle, by analyzing its effect
on the spectral properties of a generic two-level system. We will show that
spectral lines shapes are modified, if the superposition principle is violated,
and we quantify the magnitude of the violation. We show how this effect can be
distinguished from that of standard environmental noises. We argue that
accurate enough spectroscopic experiments are within reach, with current
technology.
