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]

WordNet (r) 3.0 (2006):

observingly
    adv 1: in an observant manner [syn: observantly,
           observingly]