Dim wSheet As Worksheet
On Error Resume Next
Set wSheet = Sheets("Sheet1")
If wSheet Is Nothing Then 'Doesn't exist
MsgBox "Worksheet does not exist", _
vbCritical,"OzGrid.com"
Set wSheet = Nothing
On Error GoTo 0
Else 'Does exist
MsgBox "Sheet 1 does exist", _
vbInformation,"OzGrid.com"
Set wSheet = Nothing
On Error GoTo 0
End If
There is more at: http://www.ozgrid.com/VBA/IsWorkbookOpen.htm