I've never been asked, but I've always had a return or outward bound ticket. I wonder if only having a one-way ticket sets up a notification on the computer for someone with the airlines to ask before allowing the passenger to board. There's been a bunch of past threads about this subject here and I'm sure other sites. Can't be sure but I think I remember reading that it also may depend on the airline you're flying. I believe it was mentioned Spirit, in their quest to squeeze every centimo possible, won't let you board without proof of onward travel, forcing you to buy another ticket if you want to board the flight. Getting a bus ticket from Peru>Ecuador, Tacna>Arica, etc. would seem a cheap insurance policy against potential last minute hassles at the airport. It's also been suggested that you can make a reservation for onward travel, print it out to show anyone who asks, and then never follow through and purchase the ticket, or buy a refundable ticket.