Having a soil test done for your lawn is good medicine. Just as you would not want your doctor prescribing medicine for you without checking you over, a soil test helps both you and your lawn service know that the nutrient program your lawn will receive is what it really needs. For a nominal fee, they check the primary nutrient levels, organic matter levels and soil acidity (the pH). Some tests will include other factors at an additional charge. When the report returns, you should get a copy of the report along with an analysis of the report and how your service intends to modify your program, if necessary, to adapt to your soils unique needs.
What about a do-it-yourself soil test kit? Most professionals agree that the kits you can purchase at hardware stores and garden centers are simply not accurate. For what a professional soil test will cost, you are better off having it done for you and sent out to a reputable lab.
Soil tests don't have to be done every year, but the more frequently they are done, the better you'll be aware of what needs to be done for your lawn, trees and flowers. For each of these plant groups, have a separate analysis done. Each type of plant is growing in a unique soils and has unique nutrient needs. Don't guess! Soil test!