The Collaborative International Dictionary of English v.0.48:

Diagnostic \Di`ag*nos"tic\, a. [Gr. ? able to distinguish, fr.
   ?: cf. F. diagnostique.]
   Pertaining to, or furnishing, a diagnosis; indicating the
   nature of a disease.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Diagnostic \Di`ag*nos"tic\, n.
   The mark or symptom by which one disease is known or
   distinguished from others.
   [1913 Webster]

WordNet (r) 3.0 (2006):

diagnostic
    adj 1: concerned with diagnosis; used for furthering diagnosis;
           "a diagnostic reading test"
    2: characteristic or indicative of a disease; "a diagnostic sign
       of yellow fever"; "a rash symptomatic of scarlet fever";
       "symptomatic of insanity"; "a rise in crime symptomatic of
       social breakdown" [syn: diagnostic, symptomatic]

Moby Thesaurus II by Grady Ward, 1.0:

65 Moby Thesaurus words for "diagnostic":
   characteristic, characterizing, connotative, constructional,
   constructive, contrastive, definitional, demonstrative,
   denominative, denotative, descriptive, designative, diacritical,
   differencing, differential, differentiative, discriminating,
   discriminative, distinctive, distinguishing, emblematic,
   evidential, exegetic, exhibitive, expressive, figural, figurative,
   hermeneutic, identifying, ideographic, idiosyncratic, implicative,
   indicating, indicative, indicatory, individual, individualizing,
   individuating, interpretational, interpretive, meaningful,
   metaphorical, naming, pathognomonic, peculiar, personalizing,
   proper, representative, semantic, semeiological, semiotic,
   separative, signalizing, significant, significative, signifying,
   suggestive, symbolic, symbolistic, symbological, symptomatic,
   symptomatologic, symptomatological, tropological, typical