Certain paper manufacturers seek out certifications that indicate that their products are made from 100% renewable resources. There are two main certifications available for eco-conscious paper companies: FSC and PEFC.
According to the Forest Stewardship Council, FSC certification comes from an assessment of "forest management using the FSC principles, criteria, and standards, each certifier uses their own evaluative process. This allows FSC to remain outside of the assessment process, and supports the integrity of the standard, and of the FSC system. Certifiers evaluate both forest management activities (forest certification) and tracking of forest products (chain-of-custody certification)."
Basically put, paper from an FSC certified mill is made through responsible manufacturing practices that include forests that are self-sustaining.
The PEFC is a similar certification from a different body located in Switzerland. Paper that bears either certification is assured to come from responsible manufacturing practices.