One thing very important to know about Colorado- it is the responsibilty of the hunter to make SURE you are not on private property. If hte area you hunt is pocketed with private ground, study your maps carefully, and if need be check with the locals.
Even if its an accidental trespass, the law is tough, and the fines stiff. Not to mention the locals do not take too kindly of it, regardless of your excuse.
They do not have any requirements to post their property, and just because its not posted doesnt make any difference.
That said, so landowners will try to claim part of surround NAtional Forest lands as theirs, and attempt to force you off. If you are sure you are on NF land, you have every right to be there, even if the rancher has guide permits for the same area. Some will tell you they have the hunting rights to the NF area around the ranch. All that means is they can legaly guide hunters, but doesnt mean its not still available as public hunting land.