trial balloon vs trial run

trial balloon and trial run both are nouns.

Word	Noun	Adjective	Verb	Adverb
trial balloon	Yes	No	No	No
trial run	Yes	No	No	No

As nouns, trial run is a hypernym of trial balloon; that is, trial run is a word with a broader meaning than trial balloon:

Other hypernyms of trial balloon include test, trial, tryout.

trial balloon (noun)	trial run (noun)
a balloon sent up to test air currents	trying something to find out about it
a test of public opinion