@Naj ************
it is the airline upon check-in to the flight, which MIGHT ask you for an onward travel proof within the 30 days you will get stamped in entering "visa-exempt". You could email the airline and ask, and if positive, print out their reply, as you might need it at the check-in . . . . keep in mind only 100% guaranteed proof is a "real" cheap one-way ticket out of Thailand