i have been asked every single time at check in when I'm been flying to Thailand with one way ticket. No matter where i board, Europe, asean. I have also lot of stamps in my passport without any overstay.
A lot of people claim they were never asked for onward ticket. Did you guys booked round trip with same airline.. they see the ticket from computer anyway.
Every time when I'm been flying with one way ticket, return ticket has been asked at check in desk.