That varies by state. In many places a landowner isn't liable unless he's charging for access.If you let people use your property, just make sure that your insurance coverage is up to date. If they are injured on your property, in many cases you will wind up responsible for some or all the liability. If it is posted and they trespass, they may still try to come after you, but you will be in a much better position. It is sad that this what has become of the world, but this day and age, everything has become someone else's responsibility rather than the party that should be responsible.