WA State extension doesn't test soil for gardeners, so I went with this commercial firm.
This is their instruction:
How to Test Your Soil
1. Print out the Soil Submission Form
2. Collect soil samples from the areas to be tested. Follow the simple recommendations in our Sampling Guide to obtain soil samples that are truly representative of your soil.
3. Choose a soil test option (see table below).
4. Fill out the submission form and send it together with your soil samples and a check to the address listed on the form.
Now it's just a matter of waiting a week or two for the results.
Testing for organic content, pH, phosphorus, potassium, and major and trace minerals. They don't test for nitrogen, which varies too much.