The usual way would be to have the user's Network Administrator set up access to drives/directories based on the user's rights.
In that way, if they can 'see' it, they can use it.
If they cannot 'see' it (i.e. cannot map their system to it), they cannot use it.
Another more 'kludgy' way would be to attempt a 'test' write to the directory prior to the backup write.
If the 'test' write was successful, then the backup should be as well.
If the 'test' write failed, then alert the user to try elsewhere.
Good Luck,
JRB-Bldr