These warning signs suggest stocks are heading toward a correction

Traders work on the floor of the New York Stock Exchange.

Several important market indicators are near extreme levels and that means stocks could be heading for a correction, said CFRA chief investment strategist Sam Stovall.

"It's not a question of 'if' but 'when,' the next meaningful market decline will occur," he wrote in a research note.

One of those signs is the value of S&P 500 market cap compared to nominal gross domestic product. The market capitalization, based on S&P 500 companies, is now at an all-time high of 140% of U.S. gross domestic product, compared to the near 60-year average of 62%.

The S&P 500 was down nearly 0.2% on Monday around 1:30 p.m. ET.

Stovall also expects the hardest-hit sectors in a correction could be those that have done the best over the past six months, as has historically been the case.

Those would be energy, financials and materials. But the hardest-hit sectors also bounce back more than the broader market typically.