How do I check that my features are set to their absolute height from the CDB output folder? Which are the data set I can look into?
If you are talking about a GSfeatures for example, the files NXXEXXX_D100_S001_T002_LXX_UX_RX.dbf (class attributes) will contain the attribute AHGT set to TRUE for the given feature (AHGT stands for Absolute height). The actual Z value is contained in the vector shape file at the instance level (S001_T001...shp) for that features. You can use a GIS tool to view or extract via script using shapefile libraries or using the CDB API from Presagis.
The same logic applies to GT features, just using the D101 dataset.
Where do I set it to absolute height in Terra Vista ? I want my features to use the dem as its height source but still set its AHGT to ture.
In CDB, you should avoid setting AHGT to true as much as possible. It is a client (CDB reader's) responsibility to "clamp" the object on the elevation dataset. This avoid the creation of dependencies between datasets. This way, if you update the elevation data in the CDB, you do not need to republish your 3D models to adapt to the new height. Most CDBs are published this way so most if not all client supports this.
There are specific cases where AHGT=TRUE is beneficial (ex: a bridge 3Dmodels in order to facilitate the connection with the terrain at entrance and exit) but those remain limited. you definitively should not set AHGT=TRUE on all features!
As for the way to do this in Terra Vista, it is supported, Please engage with support as this forum is about the CDB specification and not the specifics of tools operation.