USGS Benchmarks - accessing without trespassing?

[Napier]

Are United States Geological Survey benchmarks always accessible without trespassing? That is, can one assume it’s permissible to approach any benchmark? Or are they sometimes on private, inaccessable land? Is there any way to tell, other than the ways one can search any parcel of land to find out who owns it, and asking them? Are there any special rights or responsibilities associated with having a benchmark on one’s property?

[Xema]

Here’s a site on geocaching that includes this remark:

Always ask for permission if the mark you wish to hunt is on private property.
As with any outdoor activity, courtesy to the landowner is top priority. Try to determine from the location description whether or not the mark is on private property. If it is, ask for permission from the landowner. Experience shows that most landowners will give permission and many are interested in the history of the mark and the hunt and will want to help you find it.