You can easily tell if the drive is DVD-R compatible--by the logo on the front. If it doesn't have the regular DVD logo in the center of the tray with disc drawing below it and R/RW written below, but just has the RW in a square box with the words DVD+ReWritable under it, then it's not DVD-R compatible.
See, the DVD with the disc below it is a trademark of the DVD-R consortium and drives that don't have that logo haven't had copyright royalties paid to this group for the DVD-R technology--the are only authorized to write DVD+R.
But it's not big deal--in fact DVD+R is a NEWER and better format, and it's 87% compatible with set-top players. The DVD-R is maybe 92% compatible--not much more. DVD+R has better error correction capabilities as well. Just get DVD+R media and sell your DVD-R on eBay...