Tracing the contamination origins in water sources and identifying the impacts of natural and human processes are essential for ecological safety and public health. However, current analysis approaches are not ideal, as they tend to be laborious, time-consuming, or technically difficult. Disinfection byproducts (DBPs) are a family of well-known secondary pollutants formed by the reactions of chemical disinfectants with DBP precursors during water disinfection treatment. Since DBP precursors have various origins (e.g., natural, domestic, industrial, and agricultural sources), and since the formation of DBPs from different precursors in the presence of specific disinfectants is distinctive, we argue that DBPs and DBP precursors can serve as alternative indicators to assess the contamination in water sources and identify pollution origins. After providing a retrospective of the origins of DBPs and DBP precursors, as well as the specific formation patterns of DBPs from different precursors, this article presents an overview of the impacts of various natural and anthropogenic factors on DBPs and DBP precursors in drinking water sources. In practice, the DBPs (i.e., their concentration and speciation) originally present in source water and the DBP precursors determined using DBP formation potential tests—in which water samples are dosed with a stoichiometric excess of specific disinfectants in order to maximize DBP formation under certain reaction conditions—can be considered as alternative metrics. When jointly used with other water–quality parameters (e.g., dissolved organic carbon, dissolved organic nitrogen, fluorescence, and molecular weight distribution) and specific contaminants of emerging concern (e.g., certain pharmaceuticals and personal care products), DBPs and DBP precursors in drinking water sources can provide a more comprehensive picture of water pollution for better managing water resources and ensuring human health.