Man-in-the-Middle (MITM) attack enables an attacker to monitor the communication exchange between two parties by directing the traffic between them to pass through the attacker's machine. Most existing schemes for discovering MITM attack focus on detecting the mechanism used to direct the traffic through the attacker machine. This paper presents a new detection scheme that is based on matching the payload of frames exchanged in the network. The proposed scheme is independent of the mechanism used to launch the MITM attack. Experimental result shows that the proposed scheme can achieve excellent detection performance with proper choice of the scheme's tuning parameters.