f=0
for each sh in thisworbook.worksheets
if sh.name = SearchName then f=true: Exit for
next

Private Function WorksheetIsExist(iName$) As Boolean
'***********************************************'
'   Дата создания 01/01/2005                    '
'   Автор Климов Павел Юрьевич                  '
'   http://www.msoffice.nm.ru                   '
'***********************************************' 
    On Error Resume Next
    WorksheetIsExist = IsObject(Worksheets(iName$))
End Function

Private Function WorksheetIsExist(iName$) As Boolean
    On Error Resume Next
    WorksheetIsExist = (TypeOf Worksheets(iName$) Is Worksheet)
End Function

Private Function WorksheetIsExist(iName$) As Boolean
    On Error Resume Next
    WorksheetIsExist = (TypeName(Worksheets(iName$)) = "Worksheet")
End Function

Private Function WorksheetIsExist(iName$) As Boolean
    On Error Resume Next
    WorksheetIsExist = (VarType(Worksheets(iName$)) = vbObject)
End Function

Private Function WorksheetIsExist(iName$) As Boolean
    On Error Resume Next
    WorksheetIsExist = Len(Worksheets(iName$).Name) > 0
End Function

Private Function WorksheetIsExist(iName$) As Boolean
    On Error Resume Next
    WorksheetIsExist = Worksheets(iName$).Index > 0
End Function
'Пример вызова любой из вышеопубликованных авторских функций :
Private Sub Test()
    MsgBox WorksheetIsExist("Имя_Рабочего_Листа")
End Sub