After you are familiar with your garden's needs, you can have your soil tested less often perhaps every three to five years. There are several soil test kits available commercially. Usually they consist of a set of chemicals either as liquids in dropper bottles or as powders in capsules. The chemicals are reacted with a solution containing a soil sample and, depending on the resulting color, the approximate concentration of the element being tested is determined. It is important to follow the instructions precisely.
Contaminants can result in erroneous readings.
They are also available through many mail-order garden catalogs. The home gardener only has his eyes to compare the color of the solution to a color chart. By my eye, the solution was a darker green than the color corresponding to a pH of 7.0 but not dark enough for a pH of 7.5. The darkness of the solution appeared to vary depending on the angle it was viewed at and the relative lightness of the background. Similar tests can be performed to determine approximate levels of Nitrogen, Phosphorus, and Potassium.