Growing Vegetables West of the Cascades is one of my favorite books and I am definitely an obsessive fan of Steve Solomon, the author. In a section titled "What Our Soils Need to Grow Vegetables," he writes...... "The first idea I want to disabuse you of is that you need to pay for a soil test. such tests are largely useless, as most soil testing does not provide information that a nutrition-minded gardener can really use. Your money would be better spent on buying soil amendments that you'll certainly need anyway. All of our regional soils are deficient in almost everything except potassium. These deficiencies range from moderate to extreme. It's best simply to assume that this is so and add to the soil what the vegetables need to grow well. Only if you were farming significant acreage and could, with soil testing, reduce your inputs by the amounts already present in the filed, might a soil test save more than it would cost."
So there you have it. In small urban plots, it is particularly unnecessary. That said IF YOU MUST test your soil so you feel better, and you like crunching numbers and sorting through data, then I would recommend sending in a sample to the UMass's Extension program. They have a great lab and will turn around results quickly. Their site also gives you an informational step by step how-to. I will warn you, they slow down in spring and fall, so try testing as soon as you can in spring for best and fast results.