Opening Excel - Warn if read only

Hi, i need to find a way to open an excel document but warn the user if it has opened as read only.

Currently a user clicks on a button in a GUI to open an excel document, this works fine but...
If someone else opens the document whilst another user still has it open, it opens as a read only version (no warnings apart from the window title)
A lot of people do not read the read only part of the window title and continue to make changes to the sheet.  This annoys them