When deployed in a real-world setting, many sensor networks fail to meet 
application requirements even though they have been tested in the lab prior to 
deployment. Hence, concepts and tools for inspection are needed to identify 
failure causes in situ on the deployment site. Tools for inspection should 
minimize the interference with the sensor network to, rstly, ensure that 
failures of the sensor network do not break the inspection mechanism, and, 
secondly, to ensure that the inspection mechanism does not change the behavior 
of the sensor network. In this position paper, we propose passive distributed 
assertions (PDA) as a novel tool for identifying failure causes. PDA allow a 
programmer to assert certain predicates over distributed node states. Packet 
snif ng is used to detect failed assertions, thus minimizing the interference 
with the sensor networks.