Final answer:
Historians evaluate the reliability of a source by considering bias, author beliefs, and comparing it to other sources.
Step-by-step explanation:
Historians determine whether a source is reliable or not by evaluating the degree to which bias affects its conclusions, identifying the author's political and cultural beliefs, and comparing its conclusions to more popular sources. They also consider whether the source is a primary or secondary source, but it is not the only factor they rely on.