I recently took a Qatar Airways flight from Vienna to Ho Chi Minh City (layover in Doha). When attempting to board the flight with a boarding pass from online check in, I was informed that I may not board the plane without an onward airplane ticket out of Vietnam. I booked a ticket from my phone and then was allowed to board.
How I could have found out about this requirement sooner than at the boarding gate?
I've had valid passport and visa. I've checked the airline's website, the e-visa application website, and government websites, but I couldn't find the requirement for onward flight anywhere. Nobody asked to see it throughout my stay in Vietnam.
I would appreciate general advice on how to prevent boarding gate surprises in the future, rather than specific answer for Vietnam or Qatar Airways.
EDIT (not a duplicate of entry requirements question): While Is there any online resource that can tell me what visa rules are for passport holders of a specific country? is a very helpful resource (thanks xngtng), it addresses the issue of entry requirements, while this question is about boarding requirements. For example, in my case mentioned above, the IATA Travel Center says "Visitors with an e-visa or visa on arrival letter and not holding return/onward tickets could be refused entry." That's helpful, but ambiguous. I would like to know if the airline will require me to have the onward ticket. Also, in my case, the airline was requiring the onward ticket to be a plane ticket, which seems to be a requirement beyond what IATA Travel Center mentions.