Check Default Reference using Excel VBA

BuiltIn property returns a Boolean value indicating whether or not the reference is a default reference that can't be removed.

Sub If_Reference_Is_Default()

Dim i1
For i1 = 1 To Application.VBE.ActiveVBProject.References.Count

'Returns a Boolean value indicating whether or not the reference is a default reference that can't be removed.
If Application.VBE.ActiveVBProject.References(i1).BuiltIn = True Then
MsgBox "Default Reference : " & Application.VBE.ActiveVBProject.References(i1).name
Else
MsgBox "Not Default Reference : " & Application.VBE.ActiveVBProject.References(i1).name
End If
Next i1

End Sub