The phrase "excellent barometer" means something or someone that is very good at indicating or measuring changes or trends.
Sentences with «excellent barometer»
Body language and facial expressions are excellent barometers for picking up exactly how interested she is in having contact with you. (eharmony.com)
(You know, being greedy when others are fearful and fearful when others are greedy — demand for cash is an excellent barometer for fear in financial markets.) (monevator.com)
My DH insists he knows nothing about clothes, but he is an excellent barometer of whether something is JFE or not. (youlookfab.com)