We have written more than once that to enter Mexico you need to have a return ticket. This requirement is not optional; it is enshrined in the migration legislation of the state. Everyone entering Mexico, except residents and citizens of the country, must have a return ticket. Whether or not to ask for a return ticket depends on the decision of the individual immigration officer at passport control in Mexico. They often don't ask. But if there is demand, its absence will mean that you will not be allowed into the country.
Explanations like “I don’t know yet how long I’ll stay in Mexico, therefore I didn’t buy a return ticket.” will only make the situation worse. The tourist must know the plan of his trip and confirm it, otherwise you will not be considered a guest of power. The requirement to have a return ticket applies to the migration laws of many countries around the world. Another thing is that Mexico has recently become especially zealous about its implementation.
Should he have a return ticket only booked or already purchased? There is no formal explanation. That is, a ticket has the right to be booked, but not purchased. Considering that most airlines do not hold reservations for more than a day, there is not much difference between the reservation and the purchased ticket.
Will they check whether the ticket is actually booked and not fake? At passport control in Mexico they do not check the authenticity of the return ticket, although we do not rule out that migration officers have prospects for this. These chances are at least the same as for everyone else: check the ticket using the booking code in booking systems.
But danger has the opportunity to wait on the other side. The return ticket, as well as other confirmations of the purpose of the trip, can be checked by the airline upon boarding the flight to Mexico. Airlines act as migration agents; they are delegated the right to verify the traveler’s compliance with the migration rules of the country of arrival. If a passenger is not allowed into the country due to non-compliance with migration rules, the airline is obliged to transport him back at its own expense. Because she did not check the passenger before allowing her to board.
From airlines are personally friendly with reservation systems and successfully verify the authenticity of the reservation. American Airlines in the USA checked our ticket from Mexico to Lufthansa. They didn’t just look from, but looked for the reservation code using the system. In turn, Lufthansa checks tickets from Mexico upon landing in Frankfurt, and Turkish Airlines — in Istanbul.
There are not many reservation systems in the world — Amadeus, Travelport (Apollo, Worldspan and Galileo), Saber and Shares. If a passenger needs to know in which system or with which airline the ticket was booked to check the reservation by code, then airlines have the opportunity to carry out such verification much faster with the assistance of internal resources. If the reservation is not found, you can assume that it is not valid. The airline will come to the same conclusion if no reservation system recognizes the code as its own.
But the authenticity of tickets is not checked directly and not always. Let us add that a return ticket from Mexico can be to any country and for any type of transport. For example, if you have a US visa, look for Greyhound bus tickets from Mexico to the US. They can often be purchased for as little as $1 (Tijuana — San Diego or Monterrey-Laredo).
If you are convinced that buying a full return ticket is expensive, have one drawn. There is a possibility that no one will check it. But if they check, they will refuse to board the flight or enter to Mexico.
UPD. Now we already have confirmation that the Mexico Migration Service has permission and verifies the authenticity of the return ticket reservation when passing through passport control upon arrival.