This information is provided by the INativeType interface. You can access the native type for a given layer's data source as shown below (I hope you dont mind some VBA, should be easy to follow):
Sub DisplayLayerDataTypeString()
Dim pDoc As IMxDocument
Dim pLayer As ILayer
Dim pDataLayer As IDataLayer
Dim pDataSourceName As IName
Dim pNativeTypeInfo As INativeTypeInfo
Dim pNativeType As INativeType
Set pDoc = Application.Document
Set pLayer = pDoc.FocusMap.Layer(0)
If Not TypeOf pLayer Is IDataLayer Then
Debug.Print "Not a data layer"
Exit Sub
End If
Set pDataLayer = pLayer
Set pDataSourceName = pDataLayer.DataSourceName
If Not TypeOf pDataSourceName Is INativeTypeInfo Then
Debug.Print "Not a native type data source"
Exit Sub
End If
Set pNativeTypeInfo = pDataSourceName
Set pNativeType = pNativeTypeInfo.NativeType
Debug.Print pNativeType.Description
End Sub