1
general::
Phrase(s): the acid test
Fig. a test whose findings are beyond doubt or dispute. • The senator doesn’t look too popular just now, but the acid test will be if he gets reelected.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
A decisive trial to determine worth or quality, as in Exposure to brilliant sunlight is the acid test for showing this fabric won't fade. Alluding to a 19th-century chemical test for distinguishing gold from other metals, this term was used figuratively by the early 1900s.
American Heritage Idioms