Hi Xvtn. Forest lands come in two varieties: those where the land is actually owned by the USFS, and those where the land is considered part of the national forest but actually owned by someone else. 
https://www.fs.usda.gov/ivm/ is a map that shows all these boundaries. The darkest colors are the wilderness areas (e.g. the Wellsville Mountains), the next darkest shade is the lands owned by the USFS, and the lighter shade is lands considered part of the forst but not owned by the USFS.

Anything in land owned by the USFS is public unless designated otherwise. However, this is not the case on the lands that are part the forest but privately owned. I've been using a combination of the USFS map, streetview imagery, satellite imagery, and local knowledge in making these determinations.

Hope this helps, let me know if you have any questions.