One popular indicator is pointing to a sell-off in the Standard & Poor's 500 of as much as 5 percent, an analyst at Fundstrat said.

This measure has a near perfect track record for predicting corrections, Fundstrat's Tom Lee wrote in a note to clients last week. "If historical precedent applies, set for about a 4 percent to 5 percent decline in the next month towards 2,300 or so," he said.

He looked at the percentage of NYSE-traded stocks that are priced higher than their average closing prices for the last 200 trading days, a technical signal called the 200-day moving average. It is a standard measure technical analysts use to decide whether the market's momentum is on an upswing or a downswing.

Lee says that in 23 of the 24 instances of stocks slipping below their 200-day moving average, the S&P 500 index fell 4 percent or more.

Five year chart of NYSE stocks above 200-day average

Source: Barchart

On Friday the number of stocks trading above their 200-day moving averages fell to exactly 50 percent. This is the tipping point where Lee says the S&P 500 will begin to fall.

And history shows that when the S&P 500 does fall in these scenarios, it falls at or below its 200-day moving average the vast majority of those times.

The S&P 500's current 200-day moving average is around 2,354. It could fall to 2,300 or worse, a nearly 150 point drop from the current level of 2,445.