Investors with a lot of money to spend have taken a bearish stance on United Airlines Holdings (NASDAQ:UAL).And retail traders should know.We noticed this today when the trades showed up on publicly available options history that we track here at Benzinga.Whether these are institutions or just wealthy individuals, we don't know. But when something this big happens with UAL, it often means somebody knows something is about to happen.So how do we know what these investors just did? Today, Benzinga's options s ...