Daemon Sniffer is a utility program which reveals the daemon on a mainframe device. It costs 0 PWR to use, and has a cooldown of 3 turns. Daemon Sniffer can be bought from some Server Terminals for 350 credits.
While Mask is active, Daemon Sniffer can also be used on devices which do not have daemons installed, in which case it reveals that the device has no daemon.
Revealing daemons on devices is a very useful ability — every daemon is less dangerous if you can plan for it in advance. Nonetheless, Daemon Sniffer is not a good program, because there are better alternatives. In every facility where there are daemons installed on mainframe devices, there is also a Daemon Database which can be hacked to reveal every daemon — hacking the Daemon Database is better than using Daemon Sniffer simply because Daemon Sniffer takes up a program slot.
Of course, you can't necessarily find the Daemon Database early, and the Daemon Database may have a daemon itself when you find it. However, with good exploration, the Daemon Database covers most of the same use-cases as Daemon Sniffer. If you want a dedicated program to deal with the critical devices you need to hack before finding the Daemon Database, or the case where it has a daemon itself, you would be better served by Hunter or Taurus which can remove daemons from those critical devices, where Daemon Sniffer merely identifies them.