Click on the Apple logo on the top left of your screen. Go to "About this Mac". Click the "More Info" button. This will open up the System Profiler. Click "Disc burning" on the left hand side.
There is a line that says "DVD-Write" with all of the types of media your mac can burn. On my Mid-2007 iMac, I see both "-R DL" and "+R DL" as types of DVD media my drive can burn.