You can always visually inspect a disc. Cleaning is easy! Just make sure you wipe from center out and never wipe in the direction of the little grooves. That may clean old vinyl, but not good for discs.
Sometimes newer releases will insert "bogus" files that DVDShrink and DVD Decryptor will list as read errors. Clean the disc thoroughly then use DVDClone with AnyDVD to create your back up. Also DVDShrink with AnyDVD in the background usually solves the problem.
If you plan on backing up older or "well-used" DVD's, invest in an inexpensive hand crank disc cleaner. After re-surfacing this way your disc will look brand new! Usually around $20.00 USD's. Use an approved disc cleaning solution. Eyeglass cleaner will also work well.
Hope this helps.