Often the filesystem label in the ISO will tell you. You can use the file
command to see it.
$ file some_random.iso
some_random.iso: ISO 9660 CD-ROM filesystem data 'CentOS_6.6_Final' (bootable)
(I deliberately gave the file a non-descriptive name just to demonstrate.)
For a physical disk in a drive, use "file -s /dev/cdrom" (or whatever device name is appropriate).