A considerable number of Post-Tensioned (PT) bridges have been constructed because PT systems enable them to carry significant traffic loads and have an aesthetical structure. However, strand corrosion has been a long-standing issue because it may lead to the failure of tendons and the deterioration of structural performance. The corrosion typically occurs in voided locations with exposed strands; thus, the inspection of voids in external PT tendons is important and necessary in order to protect strands before corrosion occurs. Based on literature review, several Non-Destructive Testing (NDT) methods are compared for effectiveness of identifying voids in external PT tendons, and the Impact Echo (IE), ultrasonic, and sounding inspection methods are then selected and assessed using small-scale and mock-up specimens. From the experimental results, the wave-based inspection methods, IE and ultrasonic methods, are difficult to apply in the field because the imperfect bonds between ducts and grouts obstruct the transmission of waves. However, the sounding inspection method is not affected by the discontinuities and successfully identifies voids in test specimens. Thus, the sounding inspection can be an effective tool for identifying voids because of its easy application in the field.