Same way as I check CDs: Check both sides for scratches (very light ones on the bottom usually arent the end of the world), then hold it up to the light to make sure there are no holes in the top layer.
You can also try ripping them, I suppose. I use DVD decryptor to rip to the hard drive. (And DVD Shrink to make a < 4.7 GB Nero-burnable .iso image if I'm going to do a backup.)
EDIT: It occurs to me, though, that DVD Decryptor has never generated an error for me, even when I'm at a library with my laptop, so it probably just skips over the errors. (I have a rip that glitches in the same spot every time--maybe that's how disc errors manifest themselves on rips. I'm still kind of learning about this stuff.)