  We discuss a consistency test which makes it possible to discriminate unknown
nuclear background lines from neutrinoless double beta decay with only one
isotope. By considering both the transition to the ground state and to the
first excited $0^+$ state, a sufficiently large detector can reveal if
neutrinoless double beta decay or some other nuclear physics process is at
work. Such a detector could therefore simultaneously provide a consistency test
for a certain range of Majorana masses and be sensitive to lower values of the
effective Majorana mass.
