  We explain how gapped quantum spin liquids, both conventional and
'fractonic', may be unambiguously diagnosed experimentally using the technique
of multidimensional coherent spectroscopy. 'Conventional' gapped quantum spin
liquids (e.g. $Z_2$ spin liquid) do not have clear signatures in linear
response, but do have clear fingerprints in non-linear response, accessible
through the already existing experimental technique of two dimensional coherent
spectroscopy. Type I fracton phases (e.g. X-cube) are (surprisingly) even
easier to distinguish, with strongly suggestive features even in linear
response, and unambiguous signatures in non-linear response. Type II fracton
systems, like Haah's code, are most subtle, and may require consideration of
high order non-linear response for unambiguous diagnosis.
