This very simple example just looks for the workbook name:
Sub gsnu()
Dim wb As Workbook
Dim s As String
s = InputBox("enter Workbook name" & Chr(10) & "example: Book1")
For Each wb In Workbooks
If s = wb.Name Then
MsgBox (s & " is open")
Exit Sub
End If
Next
MsgBox (s & " is not open")
End Sub
Have a pleasant day!!