Sub Test()
MsgBox 'Fichier ' & IIf(FichOuvert('MonClasseur.xls'), '', 'NON ') & 'ouvert.'
End Sub
Function FichOuvert(F As String) As Boolean
Dim Wk As Workbook
On Error Resume Next
Set Wk = Workbooks(F)
On Error GoTo 0
FichOuvert = Not Wk Is Nothing
End Function