All I know is there is one thing that is consistent, some people actually time the market by it successfully. That's when the so called experts start predicting or painting the picture of a coming doom, it's safe to believe the opposite is the real truth. The markets nearly always climb "the wall of worry".
The other thing that's consistent, is when the experts start spouting that the markets are booming, and they're all saying nothing but great things are coming for the markets, that's the time to be concerned - many people use that as a good indicator to cash out.