It should be somewhere on the DVD casing if it has a region lock (not all DVDs have them).
If you have the DVD inserted and start VLC from command line it will show it. Example output:
libdvdread: Encrypted DVD support unavailable.
************************************************
** **
** No css library available. See **
** /usr/share/doc/libdvdread4/README.css **
** for more information. **
** **
************************************************
libdvdnav: DVD disk reports itself with Region mask 0x00fd0000. Regions: 2
libdvdnav: Language 'en' not found, using 'fr' instead
libdvdnav: Menu Languages available: fr
from an DVD of Elfenlied.