Here is a context clues worksheet on the noun litmus test. It means, as we use in daily discourse, “a test in which a single factor (as an attitude, event, or fact) is decisive.”
As I prepare this post, it occurs to me that I may have never used this document in a class. I prepared it to have it ready–I think this is a noun phrase students ought to know, because of the commonness of its use in even conversational English. But it is also, in its literal sense, a term of art in the physical sciences as a pH indicator when testing materials for acidityterm.
If you find typos in this document, I would appreciate a notification. And, as always, if you find this material useful in your practice, I would be grateful to hear what you think of it. I seek your peer review.