To determine a player's inability to distinguish the pitches he should be swinging at, simply take a gander at his walk-to-strikeout ratio. Pence's figure, for example, is 42:109 this season. His walk rate is under eight percent. Without getting too technical, those numbers stink.
But sometimes, walk-to-strikeout ratios and walk rates don't tell the whole story.
You see, some players have a unique ability to get decent wood on the ball wherever it's pitched. In rare cases, those players excel at the plate—like Vladimir Guerrero or, more recently, Pablo Sandoval. Because guys like that know they can get a piece of a ball if it's above their heads or at their shoe-tops, they get away with swinging at a huge number of pitches without having ghastly strikeout numbers.
Pence is sort of a "lite" version of that player type. He is pretty effective at getting his bat on the ball (for his career, he's swung and missed at about 11 percent of pitches), so he essentially swings at everything. Only, he's not nearly as good as the Pablo Sandovals of the world when it comes to squaring up balls outside the strike zone. That leads to fewer strikeouts than you'd expect, but an awful lot of weak contact.
The result of Pence's approach is that his walk-to-strikeout numbers are poor, but don't accurately reflect just how bad his plate discipline really is.
If we look at the rate at which Pence swings the bat, both at pitches inside and outside of the zone, the picture of his poor batting eye becomes clear.
In his career, Pence has swung at 31.7 percent of pitches outside the strike zone and has made contact on about 60 percent of those swings. By way of context, Pablo Sandoval hacks at 45 percent of pitches outside the zone and makes contact on an incredible 77 percent of those swings. So, Sandoval is a bigger hacker, but can get away with it because he's got enough talent and hand-eye coordination to get the job done. His career strikeout rate is under 14 percent.
On the other end of the spectrum is Buster Posey. He's swung at just 26 percent of pitches outside the zone this season and made contact on 77 percent of them. In short, he swings at fewer bad pitches, but makes contact on more of them than Pence.
This is all a long-winded way of showing that Hunter Pence has a terrible eye. He strikes out too much, walks too infrequently and swings at way too many pitches outside of the strike zone. All that adds up to a ton of easy outs. That problem has been compounded this season by an apparent decline in his ability to do anything with the bad pitches he continues to swing at.
Plate discipline almost never develops in players after they hit their mid-20s and Pence is pushing 30 already. As his physical tools decline, he'll find it harder and harder to get by as a free-swinger. He's been awful for the Giants since coming over from Philadelphia, and his plate discipline figures sure don't provide any reason to believe he'll be much better as he ages.
While the Giants' future is looking bright after sweeping the hated LA Dodgers, Pence's looks dimmer by the day.
All statistics in this article are from the wonderful Fangraphs.com.