  A simulation technique is described for quantifying the contribution of
three-body interactions to the thermodynamical properties of coarse-grained
representations of complex fluids. The method is based on comparing the third
virial coefficient $B_3$ for a complex fluid with that of an approximate
coarse-grained model described by a pair potential. To obtain $B_3$ we
introduce a new technique which expresses its value in terms of the measured
volume-dependent asymptote of a certain structural function. The strategy is
applicable to both Molecular Dynamics and Monte Carlo simulation. Its utility
is illustrated via measurements of three-body effects in models of star polymer
and highly size-asymmetrical colloid-polymer mixtures.
