Observation, experimentation, i.e., investigation of the natural world. Anyone with sufficient training and education. Absolute within the context of each religion. Truths based upon religious authority have no place in science because they are not subject to any scientific proof. Many examples exist of mixing of these two endeavors.
People seek scientific evidences in support of their religious authority.
However neither realm can make valid statements about the other, unless you hold the belief that the universe operates by magic and supernatural whim rather than by natural laws discovered by science, or that the supernatural does not exist. The root of the conflict between science and religion may be in the retreat in religious authority. Prior to science, religion was the only means available to explain virtually all observed phenomena. Mental illness, birth defects, and disease are no longer considered the mark of evil or of God's displeasure or punishment. When viewed dispassionately or pragmatically, the separation of the religious and scientific experiences seems clear and well defined.