Most people are familiar with using checksums on ISO images to validate the integrity of a downloaded disc image. Less people are familiar with how to examine the checksum once a disc is already burned however. It's really not difficult to do, and it's an invaluable tip to know. Often, a bad burn on a CD or DVD can cause subtle, frustrating, and tedious issues with a disc that could take one hours or days to ferret out. This should really become a staple in your troubleshooting process (sooner than later), to avoid a potentially unnecessary series of troubleshooting steps.
While there are many ways to do this, I prefer to use isosize and dd since they are widely available and pretty much standard on nearly all Linux distributions.
First, we grab the sector size and counts from the optical disc.
$ isosize -x /dev/cdrom sector count: 318976, sector size: 2048
Take note of these values, and we will use them in the next step. We can now use dd to extract the data from the drive and pipe it to our checksum utility (in this example I will use md5)
$ dd if=/dev/cdrom bs=2048 count=318976 conv=notrunc,noerror | md5sum 318976+0 records in 318976+0 records out 79600a0b7e5f8cc6806132dd21277cae
That's all there is to it. You should now be able to compare your checksum as would would on an ISO.