Another thing, that map saddlesore mentioned is the best map for public/private land boundaries. Lots of private in-holdings in the Uncompaghre, and in CO the owner does not have to post or fence his property, it's your responsibility to know the boundaries.
Likewise, you'll see lots of public land with fences for cattle that are grazed there. If you don't have a map showing public/private boundaries, you'd think those areas are private, but they're not, just fenced.
I'll add this for guys who have the same questions except for Idaho.
ID has a unique trespass law. If private land is range land, not cultivated or irrigated, the landowner MUST post it or it's open to hunting without getting permission. The regulations have specific rules about what constitutes legal posting. Fencing doesn't count because the FS and BLM fence a lot of public land for grazing. The posting rules are quite simple so they're not burdensome for land owners.
