It very much depends on the airline and outstation. I've been asked for onward tickets by Asiana in New York, Cebu Pacific in Hong Kong and Singapore and by Cathay in Hong Kong when flying into both Cebu and Manila. Incidently, Cathay in London have never asked me for the onward ticket and most other airlines haven't.
In the Cathay Pacific situation in Hong Kong I had to argue a little as my return flight wasn't a confirmed ticket, and then with Asiana in New York I bluffed it and told her I was joining a ship and showed my 'Seafarers Discharge Book' which worked.
