The Federal Bureau of Land Management (BLM) produces maps that show open trails in western states.
Since 2011, they provide “Off-Highway Vehicle Route Supplement” maps by Field Office sub regions. All open trails (with trail numbers) are identified.
These maps do not have contour lines or other details and should be used with other maps like the BLM Surface Management Status maps.
All national forests are required to publish a “Motor Vehicle Use Map” (MVUM). The maps are little more than line drawings of the trails with trail numbers.
They show the major paved road to help orient you.
In California, the California Trail Users Coalition (CTUC) publishes maps with the MVUM over laid on more fully featured maps for $3.00.
CTUC Web site
Obtain several maps so you know where to find legal trails.
Another good resource are trail guides.