l-Amino acid oxidase (LAAO) is widely distributed in many different organisms and found to play important biological roles, thus attracting a great deal of attention for characterization of its activity. Diverse detection methods with their own properties have been established. This review advanced different LAAO activity assays based on substrate consumption, cofactor amount, and product accumulation. The description of benefits and drawbacks of each method is expected to help researchers find appropriate detection method of LAAO activity for their own purpose.