Disc rot is something that's spoken of in hushed tones; you'll typically find discussions regarding laser rot, an issue that plagued LaserDiscs. When you do come across a discussion of disc rot, it's typically dismissed as a myth, something that plagued early CDs and is no longer an issue.
The truth, of course, is far different. If you've ever had a disc that was largely scratch free and still wouldn't play, it's unlikely that you held the disc up to the light and checked for disc rot.
An example of disc rot on my copy of Jet Grind Radio.
Checking for disc rot is simple; hold the disc up to a bright light. If you see light shining through, as in the image above, congratulations are in order; you have disc rot. Disc rot does not always render a disc immediately useless. If the disc has redundant data, the damaged section could be found in another area of the disc. Unfortunately, once you have disc rot, it can and likely will continue to spread, eventually rendering the disc useless. Disc rot stems from manufacturing issues, and there's nothing you can do.
Out of the collection I currently have with me, 29 discs, four are afflicted with disc rot. They remain playable thus far, but who knows for how much longer. While four out of 29 isn't a terrifying amount, about 14%, I have gone through three different copies of Code: Veronica that have had this issue, and one of the affected discs happens to be... the first disc of Code: Veronica.
Disc rot poses a problem to collectors, especially in the age of eBay. Most sellers do not check for disc rot, and are unaware of it. My copies of Code: Veronica that had disc rot came from eBay sellers, who listed the disc as mint condition. This wasn't a lie, but disc rot can affect the game at any point. As a collector, your best course of action is politely emailing the seller and asking them to check for the problem.
While I don't completely condone Dreamcast piracy, this issue should be of utmost importance. GD-ROM discs were not manufactured for a large amount of time, and the process was likely never entirely perfected. Who knows where original GD-ROMs will stand in terms of disc rot in 20 years? It's especially important for rippers to create GDR images that contain all of the original data, and creating perfect rips where possible.