The hardest part of investing is knowing when to sell a stock. When you do buy one, always have an exit strategy—not for right away, this quarter, this year or even the next few years. But constantly question how soon the seeds of destruction will bloom at your company. I guarantee they’ve been planted already. Let’s probe Apple , Facebook , Amazon and Google, because they make up about 12% of the S&P 500—while representing almost all of the stock-market’s gains this year. Yes, calls to break...