The Wireless Sniffer is an item added by the Wireless Redstone mod that "sniffs" for activated signal frequencies. This is useful when configuring Wireless Receivers. The Wireless sniffer can also sniff out open doors. When a Wireless Redstone frequency is active, the red boxes assigned to that frequency lights up. You can view that frequency number by mousing over the box. There are 5,000 frequencies available.
This can be useful for checking to see if a frequency is in use or not. Another method of this is the Private Sniffer, as it can reserve frequencies for the player, and tell if the frequency is in use. However it does not show reserved frequencies by you but reserved frequencies by others are greyed out.