Final answer:
To determine if a building is 'historic', one should check if it's on the National Register of Historic Places, consult local preservation commissions, and assess its architectural and historical significance following NHPA guidelines.
Step-by-step explanation:
The best way to determine if a building is "historic" and eligible for exemption from the substantial improvement requirement is to: check if the building is listed on the National Register of Historic Places, consult with a local historic preservation commission, and review the building's architectural style and historical significance. The National Historic Preservation Act (NHPA) demands a thorough review process by federal agencies for projects that could affect historic sites. This involves cooperation with State Historic Preservation Officers (SHPOs) and Tribal Historic Preservation Officers (THPOs) to preserve sites of cultural and historical importance, including those significant to Indigenous communities.