Excel creates a hidden file of the same base name prefixed with "~$" when it opens a file -- this is the backup copy. This file is transparent to the user and is deleted when the file is closed normally. I've been pretty successful in utilizing this in a GUI app to ensure the user has closed the file first before trying to write to it, but that's on a local machine. If you have latency issues with a remote connection, that'll make things tricky for that file as well.
NOTA BENE: If for some reason (like messing with COM and making a syntax error that crashes MATLAB with the file open) the Excel process is killed externally, the original file may be closed but the companion file in that case won't have been deleted automagically. You shouldn't run across this issue if you're only using the high level MATLAB functions and not munging directly through COM (and even there only during debugging, but something to be aware of if using this method).
I've found no way otherwise to do so other than another COM session and try to attach to/open the same file and trap error or use try...catch block in the high-level code; the task list data from the OS process doesn't have the filename(s) that the app has open, only that the process is running, etc., ...
