Note that some countries require passports to be valid for at least six months. Passport, visa and health requirements for entry into destination countries and for transit through a country are provided by the International Air Transport Association (IATA) on united.com as a courtesy and must be verified before travel.
The airline that controls the flight departing to an international destination has the ultimate obligation to check all immigration and passport visa requirements for the destination country must check full validity in accordance with what is published by International Air Transport Association (IATA) using TIMATICWEB. (Agents on connecting flights are not, AFAIK, obliged to do so.)
Failure to do so can result in a scenario like this: friends of mine flying on AA miles in CX Business flew AA SMF-LAX and then CX onward to HKG and DEL. On arriving at DEL, an anomaly (name error) was discovered in her visa, invalidating the visa. Cathay was required to transport her back on the return flight out of DEL at their expense (and making CX subject to a fine iirc up to $10,000). This is why airlines are pretty avid at checking docs - an employee allowing an improperly documented passenger to board can be disciplined for such an event. But the passenger bears the ultimate responsibility.
It is ALWAYS up to the passenger to check requirements and conditions of transit and destination countries. Or, one can pay agencies to do the research and issue the visas. Would-be travelers can always check out travel documentation requirements themselves using IATA TIMATIC- one airline that makes TIMATIC available freely is United, at
https://www.united.com/web/en-US/app...aspx?i=TIMATIC.