Recent experience with the deployment of sensor networks demonstrates that it 
is far from trivial to setup a working larger-scale sensor network in the 
field. Even though simulations and experiments with lab testbeds confirmed a 
working system, subtle real-world influences lead to frequent failures in the 
field. Identifying and fixing these problems in the field is currently a 
difficult and cumbersome task due to the lack of appropriate concepts and 
tools. In this paper we address this issue by, firstly, classifying common 
problems that have been encountered during deployment. We then show that many 
of these problems can be detected by overhearing and analyzing sensor network 
traffic without need for an instrumentation of sensor nodes. Based on this 
observation, we develop a tool to inspect a deployed sensor network, 
consisting of a distributed network sniffer and a data-stream-based framework 
for online traffic analysis. We demonstrate and evaluate how this tool can be 
used to debug a typical data gathering application.