Litmus

Litmus, I've just learned, is a mixture of lichen dyes (in related news, there are lichen dyes), that have been used to test acidity since the 14th century. You may remember using this to test your spit in chemistry class. 

Metaphorically speaking, this can refer to a single question whose answer allows the asker to draw a broad conclusion. According to The Grammarist, people began using this term in a metaphorical around the 1950s. The example that the Grammarist represents my new words to live by:

For instance, if experience shows that you get along with people who like cats and that you clash with those who don’t, then you might perform a litmus test on people you meet by asking whether they like cats. Those who say no are not qualified to be your friends—no analysis or additional questions necessary.

Each year, computer scientists get together to perform a specific kind of litmus test - the Turing test. The late Alan Turing proposed a test to determine whether a computer could think. The super-simple version - one person and one computer would hold an on-screen conversation. If, after five minutes, the observer can't tell which is the human and which is the computer, the machine can be said to be "thinking." Some scientists now propose to use this litmus test to determine whether we've achieved true artificial intelligence. So far, no dice, which is just as well, considering I've just come from seeing Avengers, Age of Ultron and would prefer not to be murdered by a sentient robot, thank you.
Another type of litmus test is the Bechdel test. In this test, proposed in the Alison Bechdel cartoon shown below, a movie must have two female characters who have a conversation with each other about something other than men:

"Dykes to Watch Out For (Bechdel test origin)" by Source. Licensed under Fair use via Wikipedia - http://en.wikipedia.org/wiki/File:Dykes_to_Watch_Out_For_(Bechdel_test_origin)
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This test doesn't actually make a great gauge of whether any one movie is sexist - Interstellar fails, while 50 Shades passes. But it is shocking to see just how few movies actually do pass this test - only 3 of the 2014 Oscar nominees managed it. Only 4 of the last 14 Pixar films pass. This despite the fact that a 2013 study found that movies that pass the test do better at the box office. So while the Bechdel test might not be a great litmus test for determining whether an individual move's worth watching, it is a decent litmus test for determining the value that Hollywood places on relationships between women. 

And you thought litmus tests were just for spit.