trial balloon

noun2

1. •
   (figurative) An idea, suggestion, or prospective action, product, etc., offered to an audience or group in order to test whether it generates acceptance or interest.

   • Possibly the story is given out tentatively by way of testing the public mind—as a species of "trial balloon," if we may borrow a French term for this sort of experimentation with public sentiment. — 1908
2. •
   (historical) A small balloon released into the sky to determine the direction and tendency of winds in the upper air before a manned ascent in a larger balloon; a ballon d'essai.

   • For many weeks in succession the little trial balloons thrown up to show the course of the wind were driven back upon the shores of France. — 1870