A crucial test. A very severe and undeniable one. The allusion is to a fancy of Lord Bacon's, who said that two different diseases or sciences might run parallel for a time, but would ultimately cross each other: thus, the plague might for a time resemble other diseases, but when the bubo or boil appeared, the plague would assume its specific character. Hence the phrase instantia crucis (a crucial or unmistakable symptom), a crucial experiment, a crucial example, a crucial question, etc.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894