The Collaborative International Dictionary of English v.0.48:

Diagnose \Di`ag*nose"\, v. t. & i.
   To ascertain by diagnosis; to diagnosticate. See
   Diagnosticate.
   [1913 Webster]

WordNet (r) 3.0 (2006):

diagnose
    v 1: determine or distinguish the nature of a problem or an
         illness through a diagnostic analysis [syn: diagnose,
         name]
    2: subject to a medical analysis