Hi @razorkite! What a good questions you have there! It cannot be argued that soil is the most important component in a successful garden.
Soil is usually evaluated on fertility and texture. To be able to analyze the quality of the soil, you need to dig down about a foot using a shovel. See the cross section of the soil that you are working with.
The darker, richer upper portion is the topsoil while the next layer is the subsoil. These are both very important to have healthy plants especially the [[subsoil[[. The lower layer is what decides not only how rich the topsoil layer is but also tell how well it performs soaking up and imparting nutrients.

If the color of the subsoil is a rich reddish or brown color and easily crumbles in your finger like cookie dough, it is a good soil.
If you cannot easily push your fingers into the soil down to the last knuckle then you should loosen it. You can do this by a short tine till. Just go down about three inches to unpack the topsoil and loosen it up.

Unhealthy soil are bluish or gray in color and indicates poor drainage that prevents the nutrient and water from flowing freely through the topsoil and into the subsoil. The water then has the tendency to stagnate and rob the topsoil of its nutrition and in the process chokes the roots of the plants above.

A soil should be light enough to allow plant roots and water to easily penetrate but not so loose that water is not held by the soil. Soil which has too much clay tends to prevent water drainage and soil with too much sand allows water and nutrients to wash away too quickly.
A good gardening soil should have organic matter to provide the nutrients and micro organisms for plants health.
The only way to know absolutely what nutrients your soil has is to have a soil test. You can have soil tested at many garden centers and extension service locations. Find an extension service here: http://www.csrees.usda.gov/Extension/index.html
A soil test can also tell you if your soil is too acidic or alkaline which can be a problem for many plants. Organic matter can provide many nutrients, help with water holding or drainages, and moderate pH so adding organic matter is usually the best thing for correcting or improving soil.