It can definitely be difficult sometimes. Generally if you are reasonably close, and not in someone's front yard, nobody will care, if you explain you THOUGHT you were on BLM land.
You can look online for BLM land as well, and overlay it with city/county roads, and also get lat/long gps coordinates from the geocommunicator website, so if you have a GPS, it can help you verify BLM borders. Obviously don't rely on the GPS alone to get you there, since phone maps and gps maps for directions tend to be abysmal out on BLM land roads. But the coordinates can definitely help.
Also, I agree stopping into the local BLM office for the area you are visiting is almost always a plus. It helps avoid the rangers stopping to poke at you sometimes, and they usually know the best local spots to camp(depending on what you are looking for). I've only had one BLM office visit go kinda not so great, I asked about camping spots, and they said anywhere, and really just wanted me gone. But that was a fluke, normally they are really nice and helpful, will draw maps write directions and help find a perfect campsite; the whole thing! One time, they even offered to drive me out there (have me follow him) to the campsite area I picked!
Wish you lots of luck!
With love,
Tara