Litmus Strip

Litmus Strips, also known as Litmus Paper, are small pieces of paper, typically contained in a box, used to determine and / or locate Mary Sues, plotholes, and Out of Character Canons. In theory, litmus strips should be touched against a character in order to operate, and may take some time to register; presumably they would then change color to indicate whether or not a character is a Mary Sue or a Canon going Out of Character. This behavior has never been observed. In practice, like all HQ technology, they have a tendency to ignite even within their protective containers when exposed to particularly  offensive violations of continuity or characterisation. Such burnt litmus strips smell Bad, but at least they don't explode. Most of the time.

Litmus technology has largely been superseded with the various CADs, which can provide exact measurements from a long range. As a result, the use of litmus strips has been uncommon as early as The Original Series. Agents who repeatedly destroy CADs at a faster rate than normal, or are planning to enter a Badfic where a CAD's detonation is a given, may elect or be elected to use litmus strips as a temporary measure.

The underlying technology may also be used in some Mary Sue Litmus Tests. Rumors that the basic underlying technology is a primary component of Cafeteria food remain unsubstantiated.