The Collaborative International Dictionary of English v.0.48:

Observing \Ob*serv"ing\, a.
   Giving particular attention; habitually attentive to what
   passes; observant[1]; as, an observing person; an observing
   mind. -- Ob*serv"ing*ly, adv.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Observe \Ob*serve"\ ([o^]b*z[~e]rv"), v. t. [imp. & p. p.
   Observed ([o^]b*z[~e]rvd"); p. pr. & vb. n. Observing.]
   [L. observare, observatum; ob (see Ob-) + servare to save,
   preserve, keep, heed, observe: cf. F. observer. See Serve.]
   1. To take notice of by appropriate conduct; to conform one's
      action or practice to; to keep; to heed; to obey; to
      comply with; as, to observe rules or commands; to observe
      civility.
      [1913 Webster]

            Ye shall observe the feast of unleavened bread.
                                                  --Ex. xii. 17.
      [1913 Webster]

            He wolde no such cursedness observe.  --Chaucer.
      [1913 Webster]

            Must I budge? Must I observe you?     --Shak.
      [1913 Webster]

            With solemn purpose to observe
            Immutably his sovereign will.         --Milton.
      [1913 Webster]

   2. To be on the watch respecting; to pay attention to; to
      notice with care; to see; to perceive; to notice; to
      discover; as, to observe an eclipse; to observe the color
      or fashion of a dress; to observe the movements of an
      army; to observe an accident.
      [1913 Webster +PJC]

   3. To express as what has been noticed; to utter as a remark;
      to say in a casual or incidental way; to remark.
      [1913 Webster]

WordNet (r) 3.0 (2006):

observing
    adj 1: quick to notice; showing quick and keen perception [syn:
           observant, observing]