Are we asking how do we know when companies fail, or how do we know whether committees set executive compensation based on targets?
Executive compensation for public companies is actually public -- shareholders vote on it every year in the proxy filings. This is sometimes called "Say on Pay"
if a union of worker co-ops were to pool money, buy enough shares in a public company like apple (say 50 billion dollars worth), could they vote on Tim Cook's salary to never exceed 1 million?
yes. this is actually a very common topic to result in proxy battles around.