Investors returning from the Labor Day weekend are keeping an eye out for signs that a dizzying rally in technology stocks—and the market recovery it fueled—are losing steam.

The Nasdaq Composite slid more than 6% over two days last week after hitting a fresh high, led by Thursday’s record one-day decline in market value at Apple Inc., the most-valuable U.S. listed company. Selling intensified Friday, at one point pushing the index down almost 10% from its record and spreading to markets including crude oil and gold, before…