Highway 6, TSF Mile marker 20. There are spark arrestor checks all over. in Washington, they check everyone in the parking lot last time I was there. They also started doing heavy spark arrestor checks last year at browns camp.
Diamond Mill is a bit more relaxed though, its not as heavily patroled as say browns camp is now days and actually any time the guys patroling there have even stopped to talk to us in the parking lot at DM, they just BS they never checked spark arrestors. Browns Camp tho, they'll check and they check constantly there these days. A lot of places in washington are heavy on checking for some reason (like Jones Creek, they check everyone).
Rarely run into patrols at DM tho and DM is not many riders b/c of the single track, so usually have the trails to yourself. But, every OHV area has potential for getting checked.