Check the creature as it last existed on the battlefield to determine if it was a Human. If it was, use its toughness as it last existed on the battlefield to determine how much life is gained.
If the target creature isn't on the battlefield or is otherwise an illegal target when Death's Caress tries to resolve, Death's Caress doesn't resolve and none of its effects will happen. You won't gain any life.
If Death's Caress resolves but the target creature isn't destroyed, perhaps because it has indestructible or it regenerated, you still check if it's a Human. If so, you'll gain life equal to its toughness.