How does immigration verify the 800,000 baht requirement for visa extensions in Thailand?

IF when you got your yearly extension you were not given a paper with a date stamped on it 90 days out saying to update your bank book and come back on that date you don't need to do it at all. Your immigration office will look at the balance NEXT year when you apply for your extension and make sure you met the requirements.

I think there is only one or two immigration offices that are handing out that "come back in 90 days and show us your bank book" document. One is Jomtien and I think there's one up in Isaan that does it too

This is what the document looks like IF you have to go back in 3 months.

At your next extension or who knows if they could actually ask anytime they wanted too?

You show an updated Bank book in your name. What you do with the money after the extension Is granted is up to you

They can not check the future and will trust you to behave and follow the rules. If you apply again for a new 1 year Extension of Stay they will check if you followed the rules.

I think it's when you do your 90 day teport