There are two ways to do it. The official way is to check the BOMStructure value. The other way is checking the hidden Reference property value. This example shows both ways.
Dim oDoc As AssemblyDocument
oDoc = m_inventorApplication.ActiveDocument
' By checking the BOMStructure
If (oDoc.ComponentDefinition.Occurrences(1). _
BOMStructure = BOMStructureEnum.kReferenceBOMStructure) Then
MsgBox("Occurrence is a reference")
End If
' By checking the Reference property
If (oDoc.ComponentDefinition.Occurrences(1).Reference) Then
MsgBox("Occurrence is a reference")
End If