Trap Cancel Button Without Using MessageBox Class

Question

I have a file that opens automatically with an excelApp.Workbooks.Open statement, i.e., I'm not using MessageBox.Show. If the file is in use, I get the message, "'filenam' is being modified. Open as read-only", with OK and Cancel as options. I need to trap the "Cancel" option so I can take various actions if the user chooses this. But since I didn't use the MessageBox, class, how do I do this?