Abstract: Selenium intake is crucial for maintaining body homeostasis, and its dose-dependent characteristics influence the efficiency of selenoprotein synthesis and expression of biological functions. Consequently, precise selenium detection is vital for determining the optimal dosage. This review covers various selenium detection methods, including spectroscopy, electrochemistry, chromatography, and capillary zone electrophoresis. This review provides an in-depth analysis of the role of selenium as a key component of glutathione peroxidase, which directly scavenges free radicals, blocks carcinogenic signaling pathways through selenoproteins, and enhances T-cell activity to exert antioxidant, anticancer, and immunomodulatory effects. The strengths and limitations of various detection methods have been carefully evaluated. For instance, ultraviolet-visible spectrophotometry is known for its high sensitivity and low detection limit, making it ideal for routine analysis of food samples. Fourier transform infrared spectroscopy (FTIR) is particularly beneficial due to its non-destructive nature and its ability to distinguish between different forms of selenium. However, its sensitivity and accuracy in quantification can be influenced by sample complexity and preprocessing steps. Electrochemical methods are cost-effective and straightforward to use, making them suitable for high-throughput analysis, though they are more susceptible to interference. Chromatography, while offering high precision in selenium speciation and quantification, requires sophisticated equipment and more complex operational procedures. This review aims to provide a theoretical foundation and technical reference for developing an efficient and accurate selenium detection system and to promote further research and development of selenium-enriched foods. Future research should focus on improving the automation and sensitivity of detection methods, overcoming matrix interference in complex samples, and advancing the development of novel portable detection technologies. Furthermore, as the use of selenium in functional foods continues to rise, developing more precise selenium detection systems and deepening research into its biological effects will be crucial areas of focus.