Stocks appear stuck in a 20-point range as investors navigate ongoing tensions in Ukraine, concerns over Chinese economic growth and questions over how long the five-year bull market can last, UBS' Art Cashin told CNBC on Tuesday.
Investors pushed the S&P 500 to a resistance level between 1,886 and 1,883, with support levels ranging between 1,868 and 1,865, said Cashin, UBS' director of floor operations at the New York Stock Exchange. He contends that as market watchers debate the shelf life of the bull market that started in March 2009, the "onus is on the bulls."
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"We've got that little rectangular box that's holding us together," Cashin said on "Squawk on the Street." "We have to see if we can break out."
Traders have been talking about signs in trading volume that could signal a looming selloff—specifically negative market breadth during the last four days of positive gains, a rare occurrence that historically portends losses, Cashin said.
Such a selloff would happen immediately, he said, adding that investors should monitor yields on 10-year Treasury notes as a proxy for flights to safety.
"If nothing happens over the next three days, you can throw that out the window," Cashin said.