If you're going to judge anyone
on any one characteristic, what they choose to read
is probably your best bet.
Judge her by what she looks like? Contrary to popular opinion, there's some creedence here. Someone who cares about how they look, is clean, poised, etc, those are all things you can tell just by looking at someone. Certainly not the best way to judge someone, but it's not totally groundless.
Judge her by how pretty she is? This is probably what most people mean with the above example, but the two are really miles apart. The ideas of beauty are entirely physical, and cannot really be helped by that person. In fact, I'm more apt to dislike a girl who is drop-dead gorgeous. In my experience, most of them are as shallow as a rain puddle, and not half as interesting. This strategy, though by far the most popular, tells you nothing about the person.
Judge her by her friends? Try this: Make a list of your 3 closest friends. Now, do these people define who you are? In a small way, yes, but not completely, and almost certainly not the most important parts of who you are. Not terribly accurate, but another popular form of judgement.
Judge her by her family? I won't even bother here. Just remember that it wasn't her choice who her parents are.
Judge her by her music? Musical taste tends to vary much wider and is a lot more subjective than taste in books. It's useful in guaging someone, but there's better ways.
Judge her by her books? Now we're talking. It's a glimpse into what she chooses to do with her spare time. It's a look at what she really thinks about this world, whether she knows it or not, whether she has the courage to stand up and say it or not. It's the closest you will ever get to being behind those eyes. Pay attention to which ones have been read recently, which ones are held together with duct tape, dog-eared and ragged. Do NOT pick up books you don't know and look at the covers. BAD! So what if it's got a pony on the cover? It could still be a very good book.
Ask her what her favorite books are. Ask her if you could borrow a few. She will love you for that. And then (and here's the important part), read them. Think about them. Find the things you like about them. Favorite characters, quotes, plot lines, whatever. Do NOT say "It's a stupid talking pony! That's stupid!" You just lost a potential girlfriend. Stupid.
Read the books she loves (and, as a rule of thumb, if there's a bookshelf in her room crammed with books, there are books that she loves). Read them. If you're willing to put in that time, she'll know you're serious. If you're able to comment intelligently on them, she will love you for it. You might be the first person she's ever talked with about this stuff. Be ready for an avalanche.
I know there's plenty of girls out there who don't really read much. Am I being unfair to them? Honestly, I think I'm being more than fair. I know damn well I could never date someone who 'doesn't read much', so we're not wasting one another's time. As a friend? Sure, maybe. But not as a girlfriend, and not as a wife.
On a personal note, me and my girlfriend did exactly this, in both directions, during our awkward courting-that-I-didn't-know-was-courting-until-it-was-too-late. She borrowed a few Melanie Rawn books to me (which are great; highly recommended fantasy), and I gave her a copy of Stranger in a Strange Land. Even though she didn't like Sci-fi, and I didn't generally read fantasy, we were both very pleasantly surprised.
And, almost four years later...
Anyways, judging a woman by her books is probably your best bet, unless you have a year or two to spare to get to know eachother. If you want to save some time, and get some reading done to boot, trade books. It's easier, less embarrasing, and a whole lot less painful.