I think what we can assume is that over the long run and averaging over all public corporations, shareholders are willing and to some degree able enforce their

I think what we can assume is that over the long run and averaging over all public corporations, shareholders are willing and to some degree able enforce their interest in profit maximization. That doesn't mean it's true for every single corporation at every single moment in time. And you're right, it's impossible to know whether any individual action maximizes profits or not.