If you're investing in a dividend stock, it's important to track its performance to see if there are any warning signs that might affect its ability to pay its current rate of dividends in the future. It's those early signs that can save you from a huge disappointment later on. One stock investors have been worried about these days is UnitedHealth Group (NYSE: UNH). While it's a top health insurer in the country, rising medical costs have been weighing on its financials, and the stock price has been declin ...