Nicotinamide adenine dinucleotide (NAD), either in its oxidized (NAD+) or reduced (NADH) form, is an important substrate for hundreds of catabolic and anabolic reactions. The importance of intracellular NAD(H) metabolism to human health and disease has spurred extensive research into NAD+ biosynthetic pathways, NAD+-dependent signaling mechanisms in various model organisms, and the dynamics of the NAD+/NADH pool. However, detecting NAD+ metabolism, specifically in live cells and in vivo models, remains challenging. The traditional methods for NAD(H) measurements are either destructive, making it hard to obtain spatiotemporal information about metabolites, or cannot differentiate between NADH and NADPH. On the other hand, genetically encoded fluorescent sensors enable rapid, sensitive, specific, and real-time monitoring and quantitative assessment of subcellular NAD(H) levels, potentially overcoming the limitations of traditional techniques. This chapter summarizes the traditional detection methods for coenzyme I as well as updates about genetically encoded redox indicators for quantitative assessment of coenzyme I with real-time and spatial resolution.