The term litmus test can be literal or metaphorical.

Chemistry

Chemists use litmus to test materials for acidity on litmus paper. Litmus is a water-soluble dye extracted from certain lichens and absorbed on to filter paper; this is one of the oldest pH indicators. Litmus turns red under acidic conditions and blue under basic conditions, the colour change occurring over the pH range 4.5-8.3 (at 25°C).

The phrase litmus test is often used to describe a definite test for something, especially when such a test does not exist.

Politics

A political litmus test is a set of criteria used by a political executive -- such as a governor or president -- to evaluate an appointee to high office, particularly to judgeships.

A major issue in US presidential elections occurs when vacancies for the US Supreme Court are expected during the upcoming 4 years. Advocates for various social ideas or policies often wrangle heatedly over what sort of litmus test the president ought to apply when evaluating potential Supreme Court justices.

Discussion is sometimes spiced up if one or both sides objects to letting the label litmus test be used as a metaphor for their criteria. Also, a side might deny having any issue-based criteria whatsoever, despite the other side's claims.