Take some of your older ones out of the case & hold them up to a strong light. See if there's any pinholing. The vast majority of my CDs look okay, but some are riddled with pinholes. I've done this with other peoples CDs, too, and found an alarming amount of holes. Now this isn't necessarily audible at all, but over time the holes will get bigger & bigger until they consume the redundant areas of the discs & lead to dropouts or unplayable sections.
I have a couple extremely bad ones that I'm purposefully studying over the course of years to see how bad they get. It'll be interesting to see how long it takes before the thing won't play at all.
Sorry, this all relates to pressed CDs, which isn't the topic. But I am interested to see how other peoples CD-Rs are holding up. As I said, a lot of my old Memorex discs are unplayable now, as are some TDK & Maxells. Even some of my Sony discs that are only 2 years old are bad, although those are discs I've carried in the car.