Determine if a file is open or in use or writable

Is there a way to determine if a file is currently open in another application?

Possibly by seeing if it is writable or by some other means?

I do not want to write to it if it is available to write. I just thought maybe that was a way to detrmine if it is in use.

The reason I want to do this:

I run a timer in an AIR app and if the size of a file opened in another app has changed since the time the file was opened ( from within AIR ) then it alerts the user.

However, if the User does not change the File, the timer nevers knows when to turn off.

If there is no way way to determine if a file is open, then I would take other suggestions for this method.