Accurate runtime monitoring and testing is critical in the successful field deployment of wireless sensor networks (WSNs). Conventional techniques that demand extra reports from the sensor nodes could easily alter the node and network behaviors. The delay and asynchrony of the extra reports also implies that fine-grained monitoring can hardly be achieved. Sniff-Inside approach is presented in this case, and a non-intrusive backplane-based testing platform for wireless sensor networks is developed. With auxiliary test boards, the testing platform directly captures chip-level signals from wireless sensor nodes and the captured data are transferred through independent network paths to a monitoring server. The remote access client gains access to the test server by subscribing to test data on the server and finishes the analysis and processing of test data. It has been demonstrated that the operations of the monitored WSN can be fully identified through parsing the chip-level data across different nodes. The experimental results show that, through chip-level signal sniffing, the testing platform effectively gathers accurate runtime data with no side effects on the spontaneous behavior of sensor networks. The case study further shows the testing platform facilitates the tests of high-level functionalities, such as signal analysis, protocol verification and accurate evaluation of network performance.