That is the rub, if the owner doesn't mark it, then how would one know. That is why some jurisdictions require the marking and tossing, and others require the camper to stay clear of areas that have obvious signs, such as Ag, fencing, etc... It is pretty clear what these common law jurisdictions have in mind.

Depends also on the place. Some places are pretty much all sewn up, others have a lot of open land.