Experience suggests it could go higher. In the spring of 2007, the rate was 79.5 percent. It was above 80 percent for the better part of the 1990s.

In other words, the manufacturing sector would seem to have room to produce more stuff even without expanding factory floors or adding equipment. At Caterpillar, the giant maker of heavy equipment based in Peoria, Ill., Jim Umpleby, the chief executive, told analysts last month that, even as it closely managed costs, “we’re also preserving capacity for a potential increase in business whenever that comes.”

Economic slack is trickier to measure in the service sector; good luck judging whether, say, an insurance company is operating below or at its full potential. But one window into it is the office vacancy rate.

Nationwide, 15.8 percent of office space was vacant in the final months of 2016, according to the real estate information firm Reis. That was down from a recent high of 17.6 percent in 2011 — but could still have room to fall. In the last expansion, it fell as low as 12.5 percent, implying that millions of square feet of offices are sitting idle, ready for white-collar workers to fill them.

“Vacancy has fallen, but there are still some large blocks of space out there,” said John Ferguson, president of the Southeast division of the commercial real estate firm CBRE. “A tenant can typically find a short-term solution if they need to, even if it’s not perfect for the long term.”

One thing to keep an eye on: Development of new office space has been relatively depressed since the 2008 recession, which raises the possibility that the remaining vacant space could be filled up quickly with little new supply arriving to fulfill the demand. Mr. Ferguson sees it in Atlanta, where he is based, and in other Southern cities like Nashville and Charlotte, N.C.