  The twist angle in transition metal dichalcogenide (TMD) heterobilayers is a
compelling degree of freedom that determines electron correlations and the
period of lateral confinement of moir\'e excitons. Here we perform
polarization-resolved second harmonic generation (SHG) spectroscopy of
MoS2/WSe2 heterostructures. We demonstrate that by choosing suitable laser
energies the twist angle between two monolayers can be measured directly on the
assembled heterostructure. We show that the amplitude and polarization of the
SHG signal from the heterostructure are determined by the twist angle between
the layers and exciton resonances at the SH energy. For heterostructures with
close to zero twist angle, we observe changes of exciton resonance energies and
the appearance of new resonances in the linear and non-linear susceptibilities.
