Abstract--Monitoring the health of a sensor network is important for maintaining the health and normal operation of the network. For large-scale cost-effective monitoring, using inexpensive motes as sniffers is an attractive choice. In this paper, we quantify the capability and fidelity of mote-class sniffers for sensor network monitoring. In particular, we experimentally quantify the sustainable workload and the accuracy of delay and loss measurements by these types of sniffers. We find that (1) a sniffer can monitor traffic at the rate of 60 packets per second with little buffer overflow, (2) per-hop loss measurements from sniffers exhibit variations but are comparable to those at the receiver and (3) per-hop delay measurements from a sniffer are accurate (the errors are up to 300