Reliable sources for determining a species conservation status include: U.S. Fish and Wildlife Service, Nature Serve, The Institute for Regional Conservation, California Native Plant Society, and state natural heritage programs.
The NatureServe conservation assessment process is explained NatureServe lead botanist, Anne Frances, in the first few minutes of the video below:
https://www.plantnucleus.org/video/natureserve-network-approach-plant-conservation