  The spin-nematic state has proved elusive, due to the difficulty of observing
the order parameter in experiment. In this article we develop a theory of spin
excitations in a field-induced spin-nematic state, and use it to show how a
spin-nematic order can be indentified using inelastic neutron scattering. We
concentrate on 2-dimensional frustrated ferromagnets, for which a
two-sublattice, bond-centered spin-nematic state is predicted to exist over a
wide range of parameters. First, to clarify the nature of spin-excitations, we
introduce a soluble spin-1 model, and use this to derive a continuum field
theory, applicable to any two-sublattice spin-nematic state. We then
parameterise this field theory, using diagrammatic calculations for a realistic
microscopic model of a spin-1/2 frustrated ferromagnet, and show how it can be
used to make predictions for inelastic neutron scattering. As an example, we
show quantitative predictions for inelastic scattering of neutrons from
BaCdVO(PO_4)_2, a promising candidate to realise a spin-nematic state at an
achievable h\sim 4T. We show that in this material it is realistic to expect a
ghostly Goldstone mode, signalling spin-nematic order, to be visible in
experiment.
