Airlines, all of them, access a database that contains all the entry requirements… and they read what they need to check for each traveler nationality and if it applies for each visa type…. In general for Thailand a return tag is checked only for visa exempt passengers. If you have a visa you are not supposed to have an onward flight
only if he has not already paid taxes in his own country… Thailand has a double taxation treaty with most European countries… and most of them have higher tax rates than Thailand