is there any way to determine usage of a fifth wheel?

Best you can do is look at condition, inside, outside, under, on top, appliances, furnace, A/C, suspension, etc. One unit used a lot but owned by someone meticulous about maintenance may well be in better condition than one used sparingly but never maintained at all. Only the previous owner(s) know for sure and the latter kind probably will not tell all. Would be good to have someone knowledgeable about RVs to help look it over.