You can determine the source that controls the thickness of a surface body using the Thickness Mode indication combined with the Thickness field, both located in the Details view of a surface Body object. Upon attaching a surface body, the Thickness Mode reads either Auto or Manual.
In Auto Mode the value of thickness for a given surface body is controlled by the CAD source. Future CAD updates will synchronize its thickness value with the value in the CAD system.
In Manual mode the thickness for the surface body is controlled by the Mechanical application, so future updates from the CAD system will leave this value undisturbed.
A Thickness Mode will be Automatic until the Thickness is changed to some non-zero value. Once in Manual mode, it can be made Automatic once again by changing the Thickness value back to zero. A subsequent CAD update will conveniently synchronize the thickness with the value in the CAD system.
Thicknesses for all surface bodies are represented in a dedicated column on the Worksheet that is displayed when you highlight the Geometry object.