The ability of your burner (the hardware - your DVD RW drive) to burn either +R or -R format discs is determined by the manufaturer of your drive - not any DVDFab or burner settings.
The fastest way to determine what your drive will handle (aside from reading its manual - if you got one) is to download Kris Schoof's DVD Identifier (a freebie) and run a scan on a blank disc.
As well as finding out who made your blank DVD, there will be a section listing all the DVD blank formats - anything with a little green box beside it indicates a format your drive can handle.
Hope this helps.
P.S. If you've done any burns with DVDFab, the same info will be in the logs of the burns, but Kris's program is a much more elegant presentation of the information.