Xinhua/Wang Ying via Getty ImagesStifel is looking for one warning sign that will suggest stocks have peaked.Dispersion, a reflection of individual stock volatility, has peaked before "violent" swings in the S&P 500.The spread between the VIXEQ and the VIX, one measure of dispersion, is at an all-time high.The stock market has brushed off geopolitical turmoil this year to keep climbing, but one indicator is quietly warning that the bull market m…