you sign a statuary declaration saying you live at X address, simple. They do not need to "know" anything about where you live, the fact that you are saying you live there is proof enough because you are signing a legal document...
Just figured I would update this post with some results.
I got the residence certificate today at Korat immigration.
I did the "rental contract" with my missus and used that as proof of my residence, it worked no questions asked.
Well, they did initially ask for the house book but I showed the contract and she asked if i was renting, to which I said yes, and all ended well no problems at all...
It cost 500thb and no receipt was given. I didnt feel like saying anything about that. Also I saw on the desk several other certificates, each had a 500thb note attached, so I can only assume that this money does not go directly into their pockets, so thats something at least.