I asked the same question actually! I was told “consult the map every year, it changes all the time.”
This was a USFS road that had a VERY small USFS badge indicator on the post, but no clear “NO ORV” signage. Worst yet, my wife was driving and my dog was going apesh!t, while the sheriff questioned us. We were in our SXS, not the Cruiser. We had just bought it and registered that week, so that gained us credibility of we had no clue where to be. After reading us the riot act and scaring the crap out of my wife when they said, “this usually is where we call in the USFS, they impound your machine, and you go to jail and wait for appearance in federal court in GR.” They decided we would be let off with a going off trail ticket that cost $175.
Our cabin was on a road that was deemed “No ORV” and the next year opened back up. So it really is a check the map annually situation in Lake County. No issues since, but a lot of time we are in the Cruiser on trails now.