Nasdaq 6,000 for the first time ever, which was looking increasingly closer Monday morning, "would be amazing," Cashin said on "Squawk on the Street." "But it also underscores another thing ... that it's really just a handful stocks that have been leading this rally."
"It's kind of a negative divergence," he added. "You have 15 to 20 percent of the stocks making almost 100 percent of the gains. That doesn't go on forever."
Wall Street and stock markets around the world were riding rallies to start the week as investors were encouraged by centrist French presidential candidate Emmanuel Macron's victory in Sunday's first-round voting. Far-right rival Marine Le Pen, 48, came in a close second. The runoff is set for May 7.
Le Pen and the 39-year-old Macron, both outsiders, locked France's two major parties out of the presidential race there for the first time in nearly 60 years.
"Right now, we're basically taking down hedges and other things that were put on worrying about the [French] election," said Cashin, UBS director of floor operations at the New York Stock Exchange. "What a lot of people seem to be missing here is France is still a parliamentary nation. And you have two front-runners for president who basically don't have any parliamentary backing," he added.
Cashin said he's going to pay particular attention for any new provocations from North Korea, especially with the celebration there on Tuesday of the 85th anniversary of the foundation of its Korean People's Army.
"In the past they have used that for some pseudo-military operations including bomb testing," he said, citing Sunday's threat from the rogue nation that it stands ready to sink a U.S. aircraft carrier to demonstrate its military might. Two Japanese navy ships joined the American carrier group for exercises in the western Pacific.
Cashin said he's also watching for any new policy developments out of Washington as Congress comes back from its recess to a 12:01 a.m. ET Saturday deadline to continuing funding the federal government or face a shut down. He put the changes of a government shutdown at "about 20 percent."