My answer--prepare as the GM before the session.
If you know who the enemies will be, you can be more descriptive with them, and you can have an arsenal of descriptives ready to go. Losing one eye might not impact the enemy numbers wise much, but it's a way to describe things on a great hit.
I generally have several descriptives written down and ready to go depending on the type of weapon and the type of baddie.
Some of them are in my notes as
damage of between 6-10, sword: "The blade slips between the primitive
wood armor biting into flesh as he roars in pain, spittle hitting you.
He backs away, his shield now drooping."
2-3 actual damage after DR:
"Sparks fly off the stone body of the golem, the vibrations
reverberating down your arm, with just a nick in the stone to show for
These are non-called shots, in places where I think they are likely to hit. Most of them have no impact on actual numbers in Pathfinder, they are just a way to show the enemy being worn down. I look at how much they are a percentage of their actual hit points to determine how impressively I describe them.
I also throw in a few for the kill shot.
Arrow, if just a few HP from death anyway: "The arrow grazes his chin.
He drops his dagger, he clutches at the wound, and with the motion,
another wound [whatever that may be from earlier gameplay] pours forth
blood, and he topples to the dust." (This was for a vain NPC who
valued his face.)
Impressive, well over the HP needed Arrow: "The
arrow seems to sprout from center of his neck. For a moment, his jaw
works, as though he has something to say, then he falls forward, arrow
pushed through the back of his neck from his own weight."
And so forth--using whatever the party has--daggers, swords. I also like to look at the list of spells a mage might have and looked to that to add flavor. Look at what the spells actually do and come up with descriptions.
Called shots are a different matter, but you can still prepare ahead of time with descriptions, and if you don't use them one session, you can surely use them another, converting them to fit the enemies they face. Do know the rules for called shots. They are covered here. Here's something more in-depth you can use and adapt. As you can see, there's all kinds of drawbacks to called shots--and that's enough for most players not to use them, unless they have a feat for it.
Called shots are divided into three basic difficulty groups: easy, tricky, and challenging. Easy called shots represent large areas of the body, and are made at a –2 penalty. They have relatively minor effects unless a critical hit is scored or massive damage is dealt. Tricky called shots represent either smaller areas, like a hand, or areas a creature protects well, like its head. Tricky shots receive a –5 penalty, and inflict more serious consequences. Challenging called shots represent very small areas like eyes, fingers, or creatures’ necks. They receive a –10 penalty, and successful hits cause significant short-term impairment. Beyond these challenging ratings lie almost impossible called shots that receive a –20 penalty.
The link I dropped also has a chart, a very specific one, on the effects, per limb. You can certainly get descriptive without called shots. Whether or not a called shot is used in a given fight is generally a decision made by the individual players during their turn. You can rule against called shots being used at all as the GM or assign a high difficulty if it's an impossible shot (-20). YOU as the GM don't decide when it's used, but you can over-rule usage when a player decides to call a shot. However, it doesn't mean that you can't have notes with effects based on hit points and weaponry.
The arrow description of the arrow to the neck I used--that wasn't for a called shot, I just used it once certain conditions were met. The player didn't call a shot for the neck, they just happened to roll well on a kill shot with an arrow, and I opted to describe it as that they happened to hit the guy's neck. The players might not be aiming for a specific body part, but when someone gets hit, it's somewhere specific, whether it's the torso or arm, and you can describe that, even if it has no numbers impact. I like to have special descriptions for critical hits as well--so if they roll high for damage and it's a crit, absolutely, I give out some descriptive consequences. You don't need called shots for that.
If someone actually loses their shield arm though, or is totally unable to use it, their defense would be lower. So if the shield added +1 before, I just go ahead and subtract that from their AC. There's other conditions, like bleeding and the like that could result from this, but I don't like to get too number crunchy with it because it slows things down.
I say, look to the rules and make your call. Because it's always down to the GM. Spend a lot of time looking at the conditions, such as blinded, bleeding, and the like, to see if they apply. Yes, there are special feats that people get in order to invoke these conditions, but sometimes someone will invoke those through sheer luck. Get deep into those and know what they mean, but also come armed with 8-10 descriptives of hits. Put a checkmark next to any you use, and recycle/adapt the ones you didn't for later sessions.