IsLimbGone

Returns 1 if the run-on actor has lost the specified limb, or 0 otherwise. Limb codes are the same as for GetKillingBlowLimb.

Though most of Fallout's dismemberment functionality is no longer in Skyrim, decapitation still borrows heavily from it. The head is limb #1, and this function can properly test whether an actor's head has been severed.