WordNet (r) 3.0 (2006):

trial balloon
    n 1: a test of public opinion
    2: a balloon sent up to test air currents

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "trial balloon":
   barometer, bone of contention, catechism, cross-interrogatory,
   cross-question, debating point, demand, feeler, inquiry,
   interrogation, interrogative, interrogatory, issue, leader,
   leading question, moot point, pilot balloon, point at issue,
   point in question, probe, problem, query, question,
   question at issue, question mark, quodlibet, random sample, sample,
   sound, sounder, straw vote, topic, trial, vexed question,
   weather vane, weathercock