The Collaborative International Dictionary of English v.0.48:

Doubting \Doubt"ing\, a.
   That is uncertain; that distrusts or hesitates; having
   doubts. -- Doubt"ing*ly, adv.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Doubt \Doubt\ (dout), v. i. [imp. & p. p. Doubted; p. pr. &
   vb. n. Doubting.] [OE. duten, douten, OF. duter, doter,
   douter, F. douter, fr. L. dubitare; akin to dubius doubtful.
   See Dubious.]
   1. To waver in opinion or judgment; to be in uncertainty as
      to belief respecting anything; to hesitate in belief; to
      be undecided as to the truth of the negative or the
      affirmative proposition; to b e undetermined.
      [1913 Webster]

            Even in matters divine, concerning some things, we
            may lawfully doubt, and suspend our judgment.
                                                  --Hooker.
      [1913 Webster]

            To try your love and make you doubt of mine.
                                                  --Dryden.
      [1913 Webster]

   2. To suspect; to fear; to be apprehensive. [Obs.]

   Syn: To waver; vacillate; fluctuate; hesitate; demur;
        scruple; question.
        [1913 Webster]

WordNet (r) 3.0 (2006):

doubting
    adj 1: marked by or given to doubt; "a skeptical attitude"; "a
           skeptical listener" [syn: doubting, questioning,
           skeptical, sceptical]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "doubting":
   Pyrrhonic, agnostic, ambiguous, capricious, chancy, changeable,
   dicey, distrustful, doubtful, dubious, equivocal, erratic, fickle,
   from Missouri, hesitant, hesitating, in doubt, incalculable,
   indecisive, indemonstrable, irresolute, leery, mistrustful,
   mistrusting, polysemous, questioning, scrupulous, shy, skeptical,
   suspecting, suspicious, touch-and-go, unaccountable, uncertain,
   unconfirmable, unconvinced, undivinable, unforeseeable,
   unpersuaded, unpredictable, unprovable, unsure, untrusting,
   unverifiable, variable, wary, wavering, whimsical