I ask this because I just ordered an "American Pale Ale" and it's actually quite dark in color. So how can I tell a pale ale? What actually defines an ale as pale?
The "Pale" in Pale Ale is mostly historical. In the 18th century, most beers were dark due to being produced with barley malt that was kilned or roasted over wood fires. But from England emerged a new technique using pale malts, cured in coke-fuelled kilns. This applied to both ales and lagers, resulting in beers that were bronze, copper or gold—and while you wouldn't look at such colors today and consider them notably "pale," they certainly were pale in contrast to beers back then. In the turn of the 19th century and beyond, the style spread from England to the rest of Europe—
When a new brewery was built in Pilsen in Bohemia in 1842, a coke-fired kiln was imported from England. The result was the first golden lager, Pilsner Urquell, which was made possible by British ingenuity and technical advance.
Note that American Pales are distinct from India Pales as well as from American India Pales. See my answer about the differences between styles of IPAs.
The American Pale Ale (also known as APA) is normally a light-colored ale that is traditionally hoppy with light malt flavor. But its formal description is a little bit more flexible. It's defined as a very balanced style. It originates from the English Pale Ales.
The BJCP describes it as the following (short version):
Aroma: Moderate to strong hop. Citrusy hop is very common. Also, low to moderate maltiness.
Appearance: Pale golden to deep amber. Moderately large white to off-white head with good retention. Generally quite clear.
Taste: Pretty much the same as aroma.
Mouthfeel: Medium-light to medium-full body.
For more information see here!
A pale ale is traditionally an ale that is is brewed predominantly with pale malt and is hop forward. Brewers often add additional malts in order to achieve whatever it is that they're specifically trying to achieve for a given beer, and a pale ale with some caramel or crystal malt will indeed have a darker coloration though on the whole the properties of the beer may still mean that it resides within the definition of the pale ale style.