  In an electronic system with various interactions intertwined, revealing the
origin of its many-body ground state is challenging and a direct experimental
way to verify the correlated nature of an insulator has been lacking. Here we
demonstrate a way to unambiguously distinguish a paradigmatic correlated
insulator, a Mott insulator, from a trivial band insulator based on their
distinct chemical behavior for a surface adsorbate using 1T-TaS2, which has
been debated between a spin-frustrated Mott insulator or a spin-singlet trivial
insulator. We start from the observation of different sizes of spectral gaps on
different surface terminations and show that potassium adatoms on these two
surface layers behave in totally different ways. This can be straightforwardly
understood from distinct properties of a Mott and a band insulators due to the
fundamental difference of a half and a full-filled orbital involved
respectively. This work not only solves an outstanding problem in this
particularly interesting material but also provides a simple touchstone to
identify the correlated ground state of electrons experimentally.
