  The comparison of $K^+$ and $K^-$ spectra at low transverse momentum in light
symmetric heavy ion reactions at energies around 2 AGeV allows for a direct
experimental determination of the strength of the $K^+$ as well as of t he
$K^-$ nucleus potential. Other little known or unknown input quantities like
the production or rescattering cross sections of $K^+$ and $K^-$ mesons do not
spoil this signal. This result, obtained by simulations of these reactio ns
with the Isospin Quantum Molecular Dynamics (IQMD) model, may solve the
longstanding question of the behaviour of the $K^-$ in hadronic matter and
especially whether a $K^-$ condensate can be formed in heavy ion collisions.
