The Collaborative International Dictionary of English v.0.48:

Feeler \Feel"er\, n.
   1. One who, or that which, feels.
      [1913 Webster]

   2. (Zool.) One of the sense organs or certain animals (as
      insects), which are used in testing objects by touch and
      in searching for food; an antenna; a palp.
      [1913 Webster]

            Insects . . . perpetually feeling and searching
            before them with their feelers or antenn[ae].
                                                  --Derham.
      [1913 Webster]

   3. Anything, as a proposal, observation, etc., put forth or
      thrown out in order to ascertain the views of others;
      something tentative.
      [1913 Webster]

WordNet (r) 3.0 (2006):

feeler
    n 1: one of a pair of mobile appendages on the head of e.g.
         insects and crustaceans; typically sensitive to touch and
         taste [syn: antenna, feeler]
    2: a tentative suggestion designed to elicit the reactions of
       others; "she rejected his advances" [syn: overture,
       advance, approach, feeler]
    3: sensitivity similar to that of a receptor organ; "he had a
       special antenna for public relations" [syn: antenna,
       feeler]
    4: slender tactile process on the jaws of a fish [syn: barbel,
       feeler]

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "feeler":
   advance, antenna, approach, asking price, barb, barbel, barbule,
   barometer, bid, bone of contention, cat whisker, catechism,
   cross-interrogatory, cross-question, debating point, demand,
   foretaste, hint, inquiry, interrogation, interrogative,
   interrogatory, invitation, issue, kiteflying, leader, leading,
   leading question, moot point, offer, offering, overture, palp,
   palpus, pilot balloon, point at issue, point in question,
   preliminary approach, presentation, probe, problem, proffer, query,
   question, question at issue, question mark, quodlibet,
   random sample, sample, sensor, sound, sounder, straw vote,
   submission, suggestion, tactile cell, tactile corpuscle,
   tactile hair, tactile organ, tactile process, tactor,
   tentative approach, test, tester, topic, trial balloon,
   vexed question, vibrissa, weather vane, weathercock