When HOOPS Exchange loads A CAD Assembly, the import can succeed even if some components are missing.
Some readers, such as Solidworks, can even provide the complete tessellation of the model when components are missing.
Here are
See the possible way ways to identify the missing or failing part in an assembly :

Log file

The callback functions of HOOPS exchange gives a variety of message, warning and errors that describe what happens during the import operation.
User can use them to understand what part are missing or failed to load in the assembly context.

API to get dependencies list

A3DAsmGetFilesPathFromModelFile is an API that gives the list of the dependencies of an assembly and can list the missing files.

ProductOccurrence load status

You can also rely on A3DAsmProductOccurrenceData::m_eProductLoadStatus
Each part will have a status indicating if it loaded correctly or if there was a problemin our forum post.