As wireless sensor networks grow larger, more complex and their role more significant, it becomes necessary to have an insight into the network traffic. For this purpose, sniffers play an irreplaceable role. Since a sniffer is a device of limited range, to cover a multi-hop network it is necessary to consider the deployment of multiple sniffers. This motivates the research on the optimal number and position of sniffers in the network. We present a solution based on a minimal dominant set from graph theory. We evaluate the proposed solution and implement it as an extension of the 6TiSCH simulator. Our solution assumes a 50-nodes scenario, deployed in 2x2 km outdoor area, with 10% of packet drops over all channels, when 10 sniffers are used.