Discover Pass - Tahuya, Elbe, Sadie Hills, Ocult Burn, some of Ahtanum south of Hwy 12, all GREEN DOT roads AND AREAS east of FR 1703 from south of I-90 to north of Hwy 410. Basically every state lands that have use by recreationists.
NW Forest Pass - Evan's Creek, some of the campgrounds along Hwy 410, Taneum and Manashtash, and some hiking trailheads in all National Forests, but the only trails in WA that I know of needing it are Evan's Creek. You don't need it on any ORV trail in the Mt. Baker/Snoqualmie, Winatchee, Okinagan, Ginford Pincho National Forests.
Peruse every region's website to see what is required before you, ignorance will still get you a ticket.
Now for my 2 cents.
The Discover Pass is a WA state pass for DNR trails and ORV areas that mainly pays for increased law enforcement and state campgrounds (the ones you have to still pay for entry), and Fish & Wildlife law enforcement, sites and areas, that's why you'll get caught for not having one so easily. ORV tabs are suppose to exempt those vehicles with it, but people are still getting ticketed, plus the rig that got the ORV there still needs the Pass! Volunteers and Clubs are who mainly maintain the trails.
The NW Forest Pass is USFS and it pays for the trails, campgrounds and rangers to maintain them. You only need to have the Pass if parked at hiking trailheads and day-use areas. Volunteers and Clubs help the rangers maintain the trails.
Snowmobilers get double whammied by both!
If you don't mind paying $30 for each Pass every year, get both and don't worry about any of the above.
Guess what pass I have, don't have and where I go and don't go? I can still wheel/bike without either Pass on thousands of miles of roads and trails in WA. I do miss Elbe in the Jeep and Tahuya on the bike during the winter, but I don't go to either during the rest of the year anyways.