One reliable indicator is Institutional sell

In the chart below, upper part is NYA, NY exchange index. It is the index for all stocks listed on NYSE. It is still pretty far away from all time high.

Lower part shows the trend of Institutional Selling. It is inversely related to the broad market index. 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.

(chart from stocktiming)