When people write or create images or even pass laws, they usually bring their own set of beliefs and attitudes to what they make. They have a bias that affects the final product. It’s important to be able to figure out what that bias is.

Sometimes, the bias is stated clearly. The writer in the example below does not try to hide the bias of the newspaper.

Banner announcement in printed text. Reads: “Oklahoma War-Chief. The Oklahoma War-Chief will resume publication, as a breezy newspaper, Saturday, April 26th, 1884. The new management makes no promises further than the that the CHIEF will be the authorized Organ of the Oklahoma Colony interests, and will favor the opening of the entire Indian Territory to settlement. Independent, (not neutral), it will be the mouth-piece of any political or other organization; but Standard Oil Companies, Cattle Rings, Barbed-wire-fence folks and other robber monopolies will receive attention “short, sharp an decisive.” It will aim to give NEWS from all the world—but especially from Oklahoma—news commercial, social and political. Terms--$1.50 a year, invariably in advance. Advertising rates reasonable. Address W. F. Gordon, (For the present) Arkansas City, Kansas.”
Oklahoma War Chief [Announcement of resumption of the publication of a newspaper] (April 26, 1884)

In other cases, you have to figure out what the bias is by looking at the material and thinking critically about it. The person who created these illustrations for the book Conquering the Wilderness, by Colonel Frank Triplett, made the bias of the book very obvious—showing the European Americans marching steadily into the brightness of a new day and labeling it “destiny,” and showing American Indians watching the march and labeling it “the struggle against destiny.” This form of bias is often called propaganda.

Print of a hand-drawn image. A wagon train of pioneer wagons move westward across a plain. In the background is a mountain range with the sun rising over it. In the upper left is a small circular image of white explorers standing on the edge of a cliff and pointing to a river valley below. In the lower right is a circular image of men two men on a road by a tent. In the background is a body of water with a boat. The image’s title “The March of Destiny” hangs across the top of the main image on a banner.
“The March of Destiny,” in Conquering the Wilderness by Frank Triplett (1883)
Hand-drawn image of seven or so Native men in horses. They wear feathered headdresses and carry spears. They appear to be looking at something in the distance. They are on the edge of a hill in a mountainous area.
“The Struggle Against Destiny,” in Conquering the Wilderness by Frank Triplett (1883)

In other sources, a bias can be more difficult to spot. You may even have to look outside the material and learn the identity and background of the creator. The person who wrote the next example was D. L. Payne, leader of the “Boomer” movement, which was determined to get the government to break their treaties with Native American tribes and open all of “Indian Territory” to white settlement. He had a strong pro-settlement bias.

Paragraph of printed text. Reads, “I will assert, without fear of contradiction, that there never was a tribe or an Indian that owned or had a title, (in fee simple) to one acre of land west of 96. The only claim the Indians ever did possess was that of a ‘hunting permit,’ or as termed in the treaty of 1828, an outlet as far as thee then possessions of the United States went, or to Mexico. Through the stupidity, or oversight, of the Interior Department, the United States government treated for certain lands claimed by the Chickasaws, Choctaws, Seminoles and Creek Indians in March, 1866. The Interior Department treated for the lands with a view to locate other Indians and freedmen thereon, as stated in the treaties; but in after years (1876-1878) Congress passed laws virtually prohibiting its use for that purpose.”
Excerpt from “To Our Oklahoma Colonists,” Southwest Colonization Association (1882)

Finally, a bias is not necessarily a bad thing. Lewis Hine took photographs for the National Child Labor Committee. The bias of his photographs was simply that children should not be required to work.

Sepia tone image of young boys working in a textile mill. They are barefoot and climbing on yarn winding machines.
Louis Hine, “Bobb Mill No. 1,” Library of Congress (1909)