Of all GUMSHOE abilities, perhaps the most confusing is Forensic Etymology.
Wait, no, it’s Bullshit Detector. Or Assess Honesty in the more genteel environs of Trail, or Liar’s Tell in Swords. The tell-when-they’re-lying ability.
The customary use of Bullshit Detector is to confirm when a witness is being truthful so the investigators don’t need to second-guess every bit of information, or to quickly spot inexperienced liars who’ve been compelled to conceal the truth. Major villains usually have too much self-control to be caught out.
In a whodunnit, this ability warrants a little bit of attention. Here, you’ve got a limited pool of suspects, and questioning each of them is a big part of the mystery. Obviously, you don’t want Bullshit Detector to make solving the murder trivial (Player: “Did you kill Bob?” GM: “No.” Bullshit Detector: *goes bing*), but making them all immune is equally unsatisfying. Some options:
Detect Nervousness: Bullshit Detector can’t always tell when someone’s lying, but it can tell when they’re nervous. This lets the investigator know there’s something more to a particular topic (“Fred got really flustered when I asked him about the painting in the hallway”), but doesn’t give everything away immediately.
The Leveraged Clue: The suspect is controlled and unreadable – unless the investigator can crack that stony shell. In this setup, Bullshit Detector can only be used after the investigator reveals they’re in possession of some piece of information. Once the player finds the suspect’s weak spot, they can probe on other topics. (“So, Fred, it seems you were lying about not knowing the deceased. We found this old high school yearbook, and you were classmates. Close friends even. What else have you been hiding?”)
Catching Them Off Guard: It’s one thing to maintain full composure and lie through your teeth when being questioned in a semi-formal interview – it’s something else entirely to maintain that deception at all times. Maybe the investigators can’t use Bullshit Detector when directly questioning the suspect, but it works if one of them gets him drunk, or goes out shooting with him, or pretends to be a servant.