Food authenticity is one of the major issues in the mind of today's consumers. The sale of frozen-thawed fish fillets under the "fresh fillet" label is considered as a commercial fraud. However, their close sensory properties complicate the differentiation. This study focused on analyzing the composition of exudate (pressured flesh juice) in order to rapidly differentiate between fresh and frozen-thawed skinless fillets of sea bass (Dicentrarchus labrax). Protein concentration, alpha-D-glucosidase specific activity, nucleotides and related compounds (NRCs) concentration, and free calcium concentration were measured in exudates corresponding to fresh or frozen-thawed sea bass fillets. Significant increases of these four parameters were observed in exudates from frozen-thawed fillets, especially with a twofold increase in NRCs and free calcium concentrations. These results suggest that NRCs and free calcium concentrations can be promising indicators to rapidly detect mislabeling of fresh fillets.