The idea that comes to mind for me is to add a Workbook_Open macro into that shared file. The task of that macro would be to write a TXT file into the same directory with a specific name and include the UserName of the person who opened the file. A Workbook_Close macro would delete that file.

Then you can check for that file in your other macros, if it exists you know someone has it open and can read the text file to see who.