How does the US know that I left?

You provide your passport details to the airline when you depart the US. The airline transmits that information to the authorities before departure. (Occasionally, you hear about US Customs boarding a plane that is about to leave the US and detaining or arresting a passenger. This is how they know who is on board the plane.) Customs and Border Patrol details the I-94 process here: "I still have my I-94". The main way US Customs doesn’t know you left is if you were issued a paper I-94 at a land border, didn’t leave by a commercial carrier (ie drove or walked out of the US at a land border), and didn’t surrender your I-94 when you left.

If you leave through a land border, there is a sign (at least at Canadian land borders) informing you that Canadian customs will transmit your passport information to US authorities to create a record of exit.

If you exit the country with a different passport than you entered on (eg you enter on a Canadian passport, which you might do because no ESTA is required, and leave on an Australian passport because you’re traveling home to Australia and must use your Australian passport as the travel document for the flight to Australia), I don’t know if the US will properly record that you left.

However, you are right that unlike many countries, the US does not have individual exit immigration checks. Canada and the UK are two other countries that do not have exit immigration checks. The US is definitely not as careful as Australia about recording entries and exits; they don’t always know with certainty who has left the country, but they have a pretty good idea. Certainly millions of people overstay visas in the US. (The extent to which this is truly a problem is a separate political debate.)

there is a lot of misinformation going on. It used to be anyone can leave and they would not know when you left until you declare it yourself on the paper form when flying in or crossing land border. Things are much different these days since we know have computers and unlimited databases to store everything. By Plane or Cruise ship they know when you left and when you arrived by the ship’s manifest and electronic information from security checks on your passport and boarding pass. They know this on everyone including citizens. They also take a photo of you at airline check-in and keep that too for some time, non citizens about 14 days, citizens the duration of the flight out or up to 8h. Land crossings may be different but they do share info with Canada maybe even Mexico and they now have facial recognition cameras and are installing more to keep records as well. Even if you are a US citizen everyone leaving the country with more than 10KUS$ must by law declaire it at both entry and departure. They do have agents with money sniffing dogs, the TSA scanners that detect not only weapons but drugs and currency which would be enough for them to stop you at departure and may even confiscate all your money if they do not like you or you give them a hard time. TSA works closely with CBP. Only thing i think it may be virtually impossible for them to track you is if you leave by private boat but may be boarded by coast guard (certainly they will track you by radar) or you cross into mexico or Canada illegally where there is no wall.

I respectfully disagree with this accepted answer as far as U.S. citizens are concerned. I’ve traveled internationally to China as recently as five months ago and typing in my passport information on the CBP link yields no results.

This didn’t surprise me because unless they generated a record based on my airline ticket purchase, there was no electronic reading of my passport or recording of my passport number at any point leaving or returning to the U.S.

It’s my understanding that the U.S. doesn’t care about citizens leaving or returning. Furthermore, I’d even speculate they don’t care about non-citizens leaving. They only care about non-citizens entering.

Hopefully they don’t care, unless you’re a “person of interest” to someone. That may unfortunately be way too often the case now with all of the overblown NSA tracking of everyone etc. And the NSA did absorb the US Customs service, so unfortunately they might.

But for normal purposes, they only really care when someone enters, or when someone gets involved in a legally significant event, such as a crime or accident, or working for wages.

If you show up, and then never do anything legally significant, only the most paranoid elements care when/where you left.

Yes, they almost certainly do know you’ve left. The US processes passport details for all air passengers through a system called APIS, and ties that to the electronic I-94 (arrival and departure record).

You can check your US arrival and departure history online. This allows you to verify their record of your departure.