Fibre-reinforced plastic (FRP) composites are extensively used to retrofit civil structures. However, the quality and the characteristics of the bond between the FRP and the structure are critical to ensure the efficacy of the retrofit. For this reason, effective non-destructive evaluation (NDE) methods are often necessary to assess the bonding conditions. This article presents an ultrasonic technique for detecting defects at the FRP-substrate interface. The technique uses the Akaike Information Criterion, to detect automatically the onset of the ultrasonic signals, and the novel Equivalent Time Lenght (ETL) parameter, to quantify the energy of the propagating ultrasonic signals along the interface between FRP and concrete. The uniqueness of the ETL is that it is not affected by the coupling conditions between the ultrasonic probes and the structure. The proposed NDE technique has been tested numerically by performing 2D Finite-Element analysis and experimentally on reinforced concrete samples. The results show that the method is robust and cost-effective.