I don't think its necessary. If a creature is near death, that should be obvious to the characters. The players should get the sense of how damaged their foes are by the GM's verbal description. "Which orc is bleeding the most?" is a free action that shouldn't need dice. The only time I'd use any kind of perception check is when the foe is so alien that its not clear if damage is being done. For instance, I've had demons, elementals and ghosts and other non-biological creatures not show any wounds in battle. At zero HP, they are destroyed, but until then, they appear at full strength. If a PC has a reason why they could discern the being's "health", then I'd make it a roll.
As for death blow rules, I've always found them in conflict with the concept of Hit Points. If you attack a sleeping foe in my OD&D, you get Advantage to attack and double damage, but then, if they're not dead, they're awake. Yeah, I know that's problematic with the ninja who sneaks in and slits throats, but the concept of escalating HP is supposed to protect "major heroes" from that ninja. Instead, Conan awakens bleeding profusely and then chokes the ninja to death.