Trouble doesn't come until Institutional Investors start Selling
Take a look at today's chart that shows the trend of Institutional Selling.
When the trend lines are going down, like they have been, the declining selling activity allows the market to go up.
When the trend lines are going up, like in September and October, the increasing selling drives the market down.
There is a lot a data floating around about the market, but one reliable indicator is what Institutional Investors are doing ... just don't ever go against them.