  We investigate effects of disorder on the density of states, the single
particle response function and optical conductivity in multiband
superconductors with s_{+-} symmetry of the order parameter, where s_{+-} ->
s_{++} transition may take place. In the vicinity of the transition the
superconductive gapless regime is realized. It manifests itself in anomalies in
the above mentioned properties. As a result, intrinsically phase-insensitive
experimental methods like ARPES, tunneling and terahertz spectroscopy may be
used for revealing of information about the underlying order parameter
symmetry.
