The following website will show the rules and WA state parks open to metal detecting. You can print out the form (not both pages), fill it in each time you visit, and put it in one of the pay boxes. Also, some of the park rangers like to be notified when you're detecting so they can answer the questioning public that is using the park. I'm on the Kitsap Peninsula, and found most of the Rangers to be OK w/ detecting.
http://www.parks.wa.gov/publications/other/Metal%20Detecting%20brochure.pdf
http://www.parks.wa.gov/publications/other/Metal%20Detecting%20brochure.pdf