Yeah. UnitCreatureType would be the stuff you want to figure this out, providing what you're looking for is a way to either identify the target as a critter, or make something happen when targeting one.
local squishy = UnitCreatureType("target")
if (squishy == "critter") then
-- Whatever should happen here
end
(And of course a
self:RegisterEvent("UNIT_TARGET") to make it actually check for the event.
)