  A method is described for estimating effective scattering lengths via
spectroscopy on a trapped pair of atoms. The method relies on the phenomena
that the energy levels of two atoms in a harmonic trap are shifted by their
collisional interaction. The amount of shift depends on the strength of the
interaction (i.e. scattering length). By combining the spectra of the trap
state energy levels and a suitable model for the effective scattering length,
an estimate for the latter may be inferred. Two practical methods for measuring
the trap spectra are proposed and illustrated in examples. The accuracy of the
scheme is analyzed and requirements on measurement precision are given.
