How do they know? The question is, what are you going to declare and say it is? It's not as though you remit some money to Thailand and the TRD has to guess what it is. The purpose of the tax return is for you to tell them. Their job is to determine if they believe you and whether to ask for more evidence.
Good point, so I guess I report a truthful "0" earnings from 2024 was brought into Thailand. Thanks