The airline is responsible that their checked in passengers have a valid visa ( if required ) as they have to pay for the return flight in case the passenger is denied entry into the country and the airlines could and should know that the passenger did not has a valid visa.
For this reason the check-in staff will check you’re passport to see if you have a valid visa for the duration of the stay.
Also the Thai immigration can request to see the onward / return ticket when you try to enter the country. ( unless you have a non-o visa )