This is part two of a four-part blog series about Reading. We're covering, in order of increasing depth:
Metaphors for Scanning
If the skimmer water-skis, then the scanner fishes. He may not know exactly what kind of fish he wants, but he wants fish. He also isn't looking in the wrong place—he doesn't seek trout in the ocean or tuna in the lake. An even better example than fisherman is the one armed with the spearfish. She is looking for fish by swimming in the water and not floating on top of it. Flippers on her feet, snorkel in her mouth, and mask on her face, she is armed and dangerous.
Scanning for What Exactly
So, what are you looking for in a text, and how have you armed yourself? If it's a literary text, can you narrow down what you're looking for to a chapter or two? Perhaps you're looking for proof that Mr. Collins is a sycophant—first narrow down the chapters in which Mr. Collins appears, then you'll have a much easier time pinpointing his must illustrative lines. Perhaps you're looking for pertinent quotations of the main character to support your thesis; in this case, scan with a pencil in hand and mark the margin with the first letter of the character's first name every time she speaks.
Tools for Scanning
In a reference work or a history text, use the Table of Contents and the Index first. Realize that there can often be more than one index. In a Greek textbook, you'll have two dictionaries, an appendices with fully parsed grammatical forms, and often an index of proper names. One of my favorite textbooks—Chase & Philips A New Introduction to Greek—has nine appendices and a clearly laid out table of contents. This is not just a text you use once, but a reference work to sit on your shelf for decades afterwards, and the easiest work to consult when clarifying an obscure passage. History texts and monographs will also have indices that allow you to "gut" the work, seeking only the information your need. Remember, not every book on your Works Cited page has to be something you read cover to cover.
Scanning is the tool of the researcher, but it's often hard not to wander down a rabbit trail while trying to hunt for butterflies. One remedy for this is to hunt backwards. Once you know the page or pages on which your information can be found, move your eyes from the bottom of the page to the top looking for the word. This moves against the grain that your brain has trained itself to read, so you will likely be less distracted in your hunt.
Scanning works like that button on the radio: you don't know exactly what you're looking for, but you'll recognize it when you read it (or hear it).