xlvba check if sheet exists
'VBA function to test if a sheet exists in the active workbook:
Function IsSheet(n$) As Boolean
    IsSheet = Not IsError(Evaluate(n & "!a1"))
End Function
'-----------------------------------------------------------------------
'Same thing, different technique (much faster):
Function IsSheet(n$) As Boolean
    On Error Resume Next
    IsSheet = Sheets(n).Index
End Function
