trial balloon

Short definition from Wiktionary

Noun

1. 1
   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.
2. 2
   An idea, suggestion, or prospective action, product, etc., offered to an audience or group in order to test whether it generates acceptance or interest.
3. 3
   Alternative form of trial balloon.