Your ISO images, when run thru a hash function, should produce a value that matches up to the values in this table.
Just use md5sum
from the command line.
$ md5sum FC3-i386-disc1.iso
or to test the md5 of the cd1 in your cdrom drive
$ md5sum /dev/hdc (or whatever your cdrom device is)
Both should give you a result of db8c7254beeb4f6b891d1ed3f689b412
. If not, your image/disk is corrupt and you shall have to download it again.