At least for me, it's pretty easy to differentiate state-built bridges from city or county built ones. Bridges built by Washington State Highway Department (now WSDOT) before circa 1980 had unique railing styles. These days, most utilitarian highway bridges have jersey barriers for railings.
With that said, show examples you know of state-built bridges that are not, or no longer, on state highways in Washington.
Here are a few of mine:
Portal Way just south of Blaine:
https://www.google.com/maps/@48.9720562,-122.7228001,3a,75y,164.9h,77.88t/data=!3m6!1e1!3m4!1sFmAvpjfpj1J9prUIffGQew!2e0!7i13312!8i6656?hl=en
Eldridge Ave in Bellingham:
https://www.google.com/maps/@48.762667,-122.5078528,3a,75y,300.84h,64.42t/data=!3m7!1e1!3m5!1sQlelN_0kIc6kArx-Lj8EuQ!2e0!6s%2F%2Fgeo3.ggpht.com%2Fcbk%3Fpanoid%3DQlelN_0kIc6kArx-Lj8EuQ%26output%3Dthumbnail%26cb_client%3Dmaps_sv.tactile.gps%26thumb%3D2%26w%3D203%26h%3D100%26yaw%3D158.88815%26pitch%3D0!7i13312!8i6656?hl=en
N State St in Bellingham:
https://www.google.com/maps/@48.7516776,-122.4722696,3a,75y,43.5h,75.91t/data=!3m6!1e1!3m4!1sTE2dTftSt-zSO6b9fg2xxA!2e0!7i13312!8i6656?hl=en
S Yakima Ave in Tacoma:
https://www.google.com/maps/@47.2331697,-122.442852,3a,60y,354.73h,81.27t/data=!3m6!1e1!3m4!1so2jmUJ5YzYFjBpcE0DGxNw!2e0!7i13312!8i6656?hl=en
Tacoma Ave S in Tacoma:
https://www.google.com/maps/@47.2341945,-122.4402162,3a,60y,353.51h,78.9t/data=!3m6!1e1!3m4!1sT9RDXvOoKJfyCLnWi9tgzw!2e0!7i13312!8i6656?hl=en
Union Ave in Tacoma:
https://www.google.com/maps/@47.2246509,-122.4827284,3a,60y,324.58h,79.78t/data=!3m6!1e1!3m4!1sqvsBNsWugm0rh4Tdrl8-5w!2e0!7i13312!8i6656?hl=en
Mukilteo Blvd in Mukilteo:
https://www.google.com/maps/@47.9510677,-122.2862114,3a,75y,69.76h,72.26t/data=!3m6!1e1!3m4!1sU0OvV1QGrHJUUimnpOZIrQ!2e0!7i13312!8i6656?hl=en
Here is another one, Cheney-Spokane Rd in Marshall, WA.
https://www.google.com/maps/@47.5667477,-117.4931417,3a,75y,257.47h,90t/data=!3m7!1e1!3m5!1sHb9B7b0ZthxgpuCOIb2fLg!2e0!6s%2F%2Fgeo3.ggpht.com%2Fcbk%3Fpanoid%3DHb9B7b0ZthxgpuCOIb2fLg%26output%3Dthumbnail%26cb_client%3Dmaps_sv.tactile.gps%26thumb%3D2%26w%3D203%26h%3D100%26yaw%3D251.77548%26pitch%3D0!7i13312!8i6656
Was this road once under state maintenance? This bridge looks like it was built by the state.