The Collaborative International Dictionary of English v.0.48:

Discern \Dis*cern"\, v. t. [imp. & p. p. Discerned; p. pr. &
   vb. n. Discerning.] [F. discerner, L. discernere,
   discretum; dis- + cernere to separate, distinguish. See
   Certain, and cf. Discreet.]
   1. To see and identify by noting a difference or differences;
      to note the distinctive character of; to discriminate; to
      distinguish.
      [1913 Webster]

            To discern such buds as are fit to produce blossoms.
                                                  --Boyle.
      [1913 Webster]

            A counterfeit stone which thine eye can not discern
            from a right stone.                   --Robynson
                                                  (More's
                                                  Utopia).
      [1913 Webster]

   2. To see by the eye or by the understanding; to perceive and
      recognize; as, to discern a difference.
      [1913 Webster]

            And [I] beheld among the simple ones, I discerned
            among the youths, a young man void of understanding.
                                                  --Prov. vii.
                                                  7.
      [1913 Webster]

            Our unassisted sight . . . is not acute enough to
            discern the minute texture of visible objects.
                                                  --Beattie.
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            I wake, and I discern the truth.      --Tennyson.

   Syn: To perceive; distinguish; discover; penetrate;
        discriminate; espy; descry; detect. See Perceive.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Discerning \Dis*cern"ing\, a.
   Acute; shrewd; sagacious; sharp-sighted. --Macaulay.
   [1913 Webster]

WordNet (r) 3.0 (2006):

discerning
    adj 1: having or revealing keen insight and good judgment; "a
           discerning critic"; "a discerning reader" [ant:
           undiscerning]
    2: unobtrusively perceptive and sympathetic; "a discerning
       editor"; "a discreet silence" [syn: discerning, discreet]
    3: quick to understand; "a kind and apprehensive friend"-
       Nathaniel Hawthorne [syn: apprehensive, discerning]
    4: able to make or detect effects of great subtlety; sensitive;
       "discerning taste"; "a discerning eye for color"

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "discerning":
   acute, apperceptive, appercipient, argute, astute, cogent,
   farseeing, farsighted, foreseeing, foresighted, forethoughted,
   forethoughtful, gnostic, incisive, insightful, judicious, knowing,
   knowledgeable, longheaded, longsighted, penetrating, perceptive,
   percipient, perspicacious, perspicuous, piercing, provident,
   sagacious, sage, trenchant, understanding