It is true that prices today are not quite as widely overvalued as in 1999. Large technology stocks are up 350 percent this decade, the low end of the range for the hot stocks from earlier booms, which saw gains of 300 to 1,900 percent. Only a few select technology companies — mainly the internet giants — are trading close to the valuations of the dot-com era, when the average price-to-earnings ratio for tech companies hit 50. The average ratio for that sector today is 18.

However, the scale of today’s tech boom is not readily visible because much of the investment action has moved into the hands of big private players. In 1999, nearly 550 start-ups went public, and after many ended in disaster, the government tightened regulation of public companies. In part to avoid that red tape, this year only 11 tech companies have gone public. Many are raising money instead from venture capitalists or private equity funds. Venture capitalists have poured more than $60 billion into the technology sector every year for the past three years — the highest flows since the peak in 2000 — and private equity investors say there has never been a better time to raise money.

These new private funding channels are creating “unicorns,” companies that haven’t gone public but are valued at $1 billion or more. Unicorns barely existed in 1999. Now there are more than 260 worldwide, with technology companies dominating the list. And if signs emerge that the privately owned unicorns are faltering, the value of publicly owned tech companies is not likely to hold up either.

We can never know when the end will come. Still, there are three critical signals to watch for.

The first is regulation. The tech giants are seen today as monopolizing internet search and commerce, and they are angling to take over industries such as publishing and automobiles, raising alarms at antitrust agencies in Europe and the United States. Fear that new internet technologies are doing more to waste time and brainpower than to increase productivity has already provoked a backlash in China, where officials recently criticized online gaming as “electronic heroin.” A regulatory crackdown on tech giants as either monopolies or productivity destroyers could pop the allure of tech stocks.

The other signals are more familiar. Going back to the “nifty 50” stocks of the 1960s, nearly every big market mania ended after central banks tightened monetary policy and many people who had borrowed to get in the game found themselves in trouble. The dot-com bubble peaked in 2000, after the Federal Reserve had increased interest rates multiple times. The current boom will likewise be at risk if an increase in inflation compels the Fed to raise interest rates beyond the modest rise the market currently expects.

Finally, watch for tech earnings to start falling short of analyst forecasts. The dot-com boom was driven in part by increasingly optimistic predictions for technology company earnings, and it imploded when earnings started to miss badly. Investors realized then that their expectations about profits from the internet revolution had become unreal.

Of course, no two booms will unfold exactly the same way. We are now eight years into this bull market, making it the second longest in history, behind only the run-up of the late 1990s. No bull market lasts forever, and while it is clear that we are entering the late stages of this cycle, it is impossible to say whether this moment is like 1999, or 1998 — or earlier.

The clocks have no hands, and the black horsemen may appear at any time.