How can I prove I've been in one of the eligible countries for 21 days for Thailand entry?

To demonstrate that you have spent 21 days in one of the eligible countries for entry into Thailand, you primarily need your passport with visa stamps. It is now applicable for 63 countries, and you can combine visits to multiple countries on the list. If you visit a country not on the list, the 21-day count resets. There's a mention of uploading passport stamps as proof, but there are inquiries about where to submit this information on the Thai Pass website, as it does not directly request proof during the initial application.