How does Thai immigration know if someone returned to their home country after tourism?

visiting twice a year for under what was 60 days now back to 30 days will not raise any red flags for Thai Immigration as many people travel a couple of times a year . And if staying longer than 30 days and you have the right visa should not be an issue.

From your plane ticket for starters.

The obvious way Immigration knows is the stamps in your passport.

They? Anyone can spot a bullshitter.

I had a friend who dated a very hi so girl in Bangkok. He left Thailand and told her he was going home. In fact he went to Dubai for a week to watch a golf tournament. Later that year he went on holiday with his wife and kids( Thai girl had no idea he was still with his wife) when he came back she asked him did he go home. Obviously his answer was yes. She told him he went to Dubai. Shocked he said to a golf tournament. She was ok with that. Then she asked about his family holiday. Quick thinking he said he went with his sister.

She had a cousin in the airport and he got his passport travel details.

They know where you go

Thai AI system will check how many day you have spent in Thailand within 1 year before new arrival. If more than 150 days it is a reason for interview. No matter which country you stayed before. No need to visit your home country but it is better to get long term visa.

They can't know

Your passport travel history on their screen

Before they looked at the stamps in your passport, now they look at a computer screen.

Thai laws are confusing

When you complete your TDAC you will include flight details.

They would know if you arrived back into Thailand say from Vietnam. And you showed them your return flight back to your home country before you enter Thailand again.

They don't know and they don't care.

It’s called a Boarding Pass

Has your flight details on it

I don't think they care, or even know, where you're going when you leave. You might be transiting via another country rather than taking a direct flight. But what counts as "home" anyway? I travel on a UK passport, but I've lived in the Philippines for 30 years, and I call that my home.

It really doesn't matter that much. They care mostly about how much time you've spent in Thailand. If you just spent 3 months in Thailand as a tourist, and then you leave for a week and return, immigration doesn't care if you left to Vietnam or if you went home. They still know a border run to get a new stamp when they see it.