We are going to buy land (inside a fenced development).
Does anyone know how to check that the documents are for the land we want to buy and not for a piece of land in the middle of the forest?

What land documentation do you have?

 

You could try this site: - https://landsmaps.dol.go.th/#

 

There are three colours of land ownership documents. Just make sure you have the red bird.

But it is possible the development is treated as a condo????

As @Crossy says, all you need is the number in the right hand corner of the title deed, mostly 4 digits, province and district, (as long as it is chanote).

If you want to be 100% sure you can go to the local land office and ask them to come survey the property. Depending on how busy they are, it can take some time till they'll come, but you can always pay a bit more and they'll come after hours as well 

Thanks @Crossy. https://landsmaps.dol.go.th/ is a complete solution, displaying the plot with the given number on the Google map.