As organic materials, fish and its derivative products are perishable and break down rapidly and ultimately disappear. Unless they include bones or scales, the only means of identifying them in the archaeological record is through the identification of the chemical constituents that are left behind after decomposition. Fish flesh mainly consists of proteins and lipids. Since these molecules are unstable they degrade irreversibly after death or discard, losing a significant part of the original chemical elements, consequently hampering or complicating their identification. In this paper, two main types of fish products are considered: raw, salted or cooked fish, and fermented fish-based products. In the first case, the degradation of the native markers results from chemical post-depositional degradation, and eventually thermal degradation during cooking. In the second case, the biochemical processes involve bacteria that cause the rapid decomposition of fish as part of the process of forming fish sauces, such as Roman garum. To detect and identify fish products we combined experimental archaeology and organic analyses. The identification of the degradation products of cholesterol appeared to provide a strong indicator for detecting the presence of fish sauces. Analysis of samples taken from fish-salting vats located along the Atlantic coast (Marsa, Baelo Claudia, Troia, Kerlaz, and Etel) with samples taken from actual experimental garum sauces made from mackerel (Scomber scombrus), sardines (Sardina pilchardus), and/or oysters (Ostreidae), revealed the presence of common markers of fermented fish-based products. The application of the analytical methodology described here to structures involved in ancient fish sauce manufacture and storage has enabled investigation of the production of the sauces by searching directly for the fish markers on the inner sides of the containers. For the first time the addition of molluscs and fruits in the fish product has been identified as part of the process of manufacturing garum.