The electric network frequency (ENF) is likely to be embedded in audio signals when the electronic recording devices are connected to electric power lines. If an audio signal is edited, the embedded ENF will be altered inevitably. In order to assess audio authenticity, this paper proposes a new method based on the max offset for cross correlation between the extracted ENF and the reference signal. By comparing the max offsets on a block-by-block basis, we can determine whether the audio signal in question was digitally edited as well as the location at which the editing manipulation occurs. The validity and effectiveness of our method have been verified by experiments on both synthetic composite signals and real-world audio signals.