Low cost signal detection and monitoring system is designed in this paper. Wireless Transmission Monitoring is required by security agencies in order to check transmissions taking place in a particular area. In this paper Wireless Transmission Monitoring System is implemented using GNU Radio and RTL-SDR device. The implemented system is capable to monitor signals of GSM Transmission, Commercial FM Broadcast; the system is developed using Software Defined Radio based RTL-SDR hardware. Monitoring System can also be extended as a signal receiver by making certain additions in the designed model. In this paper Multiple RTL-SDR device based signal detection system is also designed. FM signals were successfully detected by the system in laboratory environment.