  Heavily electron-doped and single-layer FeSe superconduct at much higher
temperatures than bulk FeSe. There have been a number of proposals attempting
to explain the origin of the enhanced transition temperature, including the
proximity to magnetic, nematic and antiferro-orbital critical points, as well
as possible strong interfacial phonon coupling in the case of single-layer
FeSe. In this paper, we examine the effect of the various mechanisms in an
effective two-band model. Within our model, the fluctuations associated with
these instabilities contribute to different parts of the effective multiband
interactions. We propose to use the collective phase fluctuation between the
bands--the Leggett mode--as a tool to identify the dominant effective pairing
interaction in these systems. The Leggett mode can be resolved by means of
optical probes such as electronic Raman scattering. We point out that the
Leggett mode in these systems, if present, shall manifest in the Raman $B_{1g}$
channel.
