Final answer:
NADH is the reduced form of nicotinamide adenine dinucleotide (NAD), identifiable because it has one more hydrogen ion and two more electrons than its oxidized counterpart NAD+.
Step-by-step explanation:
NADH is the reduced form of nicotinamide adenine dinucleotide (NAD). We can determine this by examining its structure and its role in cellular processes. Specifically, NAD+ is the oxidized form of the molecule, which can accept electrons and a proton (a process known as reduction) to become NADH. This is evident from the fact that NADH has one more hydrogen ion (H+) and two more electrons (e-) than NAD+, as indicated in various figures referenced. Therefore, NADH represents the molecule after it has gained these particles. Furthermore, NADH is involved in the electron transport chain, where it is eventually oxidized back to NAD+, demonstrating that it was in the reduced state when it entered the chain.