Art Cashin said today's post-open lows have created key technical levels that investors need to watch.
Cashin, UBS' director of floor operations at the NYSE, spoke with CNBC's Sue Herera in the early afternoon just as stocks were moving toward the day's lows. "That little dip-down... has taken us back down to the post-opening lows. That would be around 16040 in the Dow, 1819 in the , and 4007 in Nasdaq."
He had been encouraged when stocks bounced off those lows earlier in the session. "If they test those and hold again they may try to rally. If they break we may start to get cascade selling."
Cashin also talked about a "three-point checklist" that will alert investors if the "rumor mongers" become active going into the weekend.
"You look for stocks to go down, simultaneously yields on the 10-year (U.S. note) to go down, and perhaps gold to go up. That tells you it's likely there are some rumors about that have a geopolitical nature," possibly involving Ukraine or the Middle East.
—By CNBC's Alex Crippen. Follow him on Twitter @alexcrippen.