A creature typically can't determine another creature's remaining hit points
However, a creature can come close using spells like deathwatch and detect animals and plants, and optional rules like Wound Thresholds will allow more accurate guesses as to a creature's hp, but, overall, a creature's current hp is usually information possessed only by the GM.
Since Pathfinder monsters typically have a predictable number of hp based on CR, a successful Knowledge skill check may give a canny player a means to guess at a particular monster's hit points and, further, allow the PC to glean as "a piece of useful information" how many Hit Dice the creature possesses, if the GM's comfortable revealing that as one of the creature's "special powers or vulnerabilities."
The advantage of the power word spells is that they are ranged spells with no saving throws, a virtual saving throw being provided by the creature's remaining hp. And, while I know the power word spells' disadvantages are many, a caster that knows one or more of such spells and also knows a monster's remaining hp eliminates any guesswork in a way that's superior to the typical caster knowing a monster's current actual Fort, Ref, and Will saving throw bonuses! If a hp determination technique's available, a power word spell's caster no longer need guess and can word foes with impunity. Hence, unless a special ability or spell grants such an ability, I'd advise against a house rule permitting a creature to determine another creature's hp.