litmus test vs exam

litmus test and exam both are nouns.

Word	Noun	Adjective	Verb	Adverb
litmus test	Yes	No	No	No
exam	Yes	No	No	No

As nouns, exam is a hypernym of litmus test; that is, exam is a word with a broader meaning than litmus test:

Other hypernyms of litmus test include examination, test.

litmus test (noun)	exam (noun)
a test that relies on a single indicator	a set of questions or exercises evaluating skill or knowledge
a coloring material (obtained from lichens) that turns red in acid solutions and blue in alkaline solutions; used as a very rough acid-base indicator