The Collaborative International Dictionary of English v.0.48:

diagnosing \diagnosing\ n.
   the act or process of identifying the nature or cause of some
   phenomenon.

   Syn: diagnosis.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

diagnosing
    n 1: identifying the nature or cause of some phenomenon [syn:
         diagnosis, diagnosing]