The phrase "reliable barometer" usually means something that can accurately indicate or predict changes or trends in a situation or event. Full definition
The Oscars ™ are not always the most reliable barometer of cinematic greatness. (killermoviereviews.com)
Without a Kathryn Bigelow film in the mix this year to buck any trends, we're left to fall back on the most reliable barometer of them all: And the Oscar for most sound editing goes to Mad Max: Fury Road. (slantmagazine.com)
Long considered an extremely reliable barometer of demand, wages show that the job market, though better for science Ph.D. s than for the mythic average worker, still isn't all that hot. (sciencemag.org)