diagnostic

adj

1. Of, or relating to diagnosis.
2. Characteristic of a particular disease.
3. Serving to indicate or specify a particular item or thing within a group of similar things.

   “Each language has its own distinct phonological qualities which a counter can tap into with effect. It is not necessarily specific phonemes (though these can sometimes be diagnostic), but rather the frequency and phonotactic distribution of each disparate set of phonemes that go together in the speech stream in certain recognisable ways.”

4. Intended to assess a student's preexisting knowledge on the material being taught.

   “A diagnostic assessment”

5. Characteristic of a particular taxon or clade.

noun

1. Any technique used in medical diagnosis.
2. Any tool or technique used to find the root of a problem.
3. That by which anything is known; a symptom.