(FORTUNE Magazine) – Now that the U.S. economy has once again proven the optimists right and confounded the skeptics, the question that's been festering in everyone's mind is back: How long can this keep going? And what, eventually, will make it stop? The latest numbers show the economy has grown a remarkable 3.5% so far this year while inflation is nowhere to be found--not in the labor market, not in consumer prices (still only 2.1%), not even in wholesale prices. It's now a sure thing that in February this expansion will make it into the record books as the longest in U.S. history.

But while the business cycle has been extended, it still exists, and at some point profits will fall, investment will taper off, credit will contract, and we'll have a recession. Of all the factors that can cause one, inflation is foremost on people's minds, especially on Wall Street. Says Goldman Sachs chief economist William Dudley: "It hasn't reared its head yet, but the emphasis is on 'yet.' We are finally starting to run out of labor."

Which brings us to the Fed: How long before Alan Greenspan feels it's necessary to raise interest rates again to preempt the inflationary threat? The answer, of course, is locked in Greenspan's head, and he's not talking. Greenspan, one imagines, would be an excellent poker player.

A whole universe of economists, analysts, and traders live to anticipate his next move. And since Greenspan has always been a data hound, known for his love of obscure statistics, they try ceaselessly to figure out what numbers he's looking at in the hopes of guessing how he'll behave. Not long ago, some Wall Street types thought they had cracked the code. Greenspan's decisions on interest rates, they deduced, depended on seven economic indicators--supplier deliveries, industrial production, nonfarm payrolls, durable-goods orders, motor vehicle sales, housing starts, and commodity prices.

If only it were that easy. These days, trying to anticipate Greenspan's thinking by using such a mechanistic yardstick is less likely to work than ever. As the U.S. becomes more integrated in the world economy and as financial markets and new technology take on greater importance, Greenspan's approach has gotten even tougher to pin down.

Those familiar with the way Greenspan works say that these days he's looking at things like credit spreads, yield curves, and liquidity premiums--now more relevant in understanding how much credit is available in the economy. Greenspan is constantly searching for new data, often surprising members of the FOMC (the main policymaking body of the Fed) by using little-known indicators such as the quit rate (the number of people unemployed who voluntarily quit their jobs). "Greenspan is like a doctor diagnosing a patient," says David Jones, chief economist at Aubrey G. Lanston and author of several books on the Fed. "He looks for signs of problems and then zooms in on them."

All this helps in deducing how Greenspan will deal with everyone's current worry--inflation. To identify whether price pressures are building in the economy, Greenspan is said to be looking at labor and raw-material costs. He also watches--among other indicators--the percentage change in average hourly wages from month to month, the quarterly employment cost index, commodity prices, and the producer price index. But one blip does not make an interest rate decision. When the September PPI showed a huge jump in prices, Greenspan looked at the factors behind it to judge whether there was a sudden rekindling of inflation or one-off factors at work. (It was the latter: notably higher tobacco and car prices.)

Exceptionally strong demand from consumers and business is also said to hold Greenspan's attention. He's watching motor vehicle sales, housing starts, and new orders and shipments of nondefense capital goods. "Every indicator has a story to tell," says Mike Niemira, senior economist at Bank of Tokyo-Mitsubishi. "The trick is putting them together to get a bigger picture."

So far the bigger picture has shown there is little sign of inflation. True, labor markets are tight, but you could have said the same thing six--or 12--months ago. Commodity prices, particularly those for oil, which shot up this year, now seem to have run out of steam. Even more important may be that leading indicators are pointing to a slowdown in consumer spending.

That should all be good news going into the Nov. 16 FOMC meeting, and there's certainly a case for leaving interest rates alone right now. But lest anyone become complacent, remember that Greenspan's modus operandi has always been to act preemptively against inflation, usually when the conventional wisdom is still arguing that no interest rate increases are warranted.