In California most of the BLM areas are clearly marked with signage. You'll see the brown signs on the freeways/interstates showing nearby BLM or OHV parks. Once you get to the BLM/OHV land there are signs with rules and regulations plus signs on roads and trails showing where you can or cannot camp. Dispersed Camping is the term the BLM uses for areas where you can camp where you would like.
As others have posted go to the BLM website and locate the district pages you want to travel to. There will be maps and details for each area.