What should we do about onward flight requirements when traveling to Thailand without a fixed itinerary?

Me and my partner are travelling within the next couple of weeks and we are starting in Thailand. We have read that we need to have a return flight booked or they may not let you on the flight out there… we aren’t sure where we will be going after Thailand, just taking each day as it comes when travelling, we are thinking either Cambodia, Vietnam etc… how would we get around this as we may cross the boarders via road? Any advice or self experience would be hugely appreciated.