All airlines are required by the Thai government to check for an onward ticket. However, it depends on the airline agent you get. Unless you have an onward ticket on your itinerary, most agents do check, some don't. People will say they've never been asked. Great. Does that help you?
What I do is arrive 15 minutes earlier than normal to check-in. If the agent asks for an onward ticket, I immediately go to
*************************
and order a ticket. The agents know what I'm doing but they don't care. It just satisfies their requirement. The website advertises they will issue a ticket within 60 seconds. It costs about $15US. There are a few websites that provide this service.
The tickets are automatically cancelled after 48 hours or so. By then, I'm at my final destination.