Backscatter is one of the important techniques of IoTs as it can offer low-cost and low-energy wireless communication. With the help of widely available ambient signals, backscatter communication can even work without specialized carrier generation. The current solutions, however, are unable to identify various ambient signals. So, we introduce a backscatter system to do this. In particular, we realize the identification of OFDM WiFi (802.11a/g/n), 802.11b WiFi, Bluetooth Low Energy, and ZigBee. Further, we implement our backscatter in the FPGA hardware to evaluate the design. Comprehensive field studies show that our backscatter can identify four protocols at an average identification accuracy of about 90 %. We also demonstrate that our identification is compatible with different backscatter modulation for all four signals in 2.4GHz band.