Before you boondock on their property, look behind their house and out buildings. Make sure there isn't a bunch of abandoned vehicles with out of state tags.
Then, look for possible new graves.
If you don't find any, it's probably safe to boondock on their property.
But, sleep with one eye open, and if you hear a chainsaw and see no woodpile, maybe it's time to leave. :B
If someone allows me to boondock on their property, I assume they want me there and are decent people. I wouldn't worry.