is there an fsck for .iso disk images?

I've got an isovfy utility. From the man page:

isovfy is a utility to verify the integrity of an iso9660 image. Most of the tests in isovfy were
added  after  bugs  were discovered in early versions of mkisofs. It isn't all that clear how useful
this is anymore, but it doesn't hurt to have this around.

Maybe that will help?

The usual way to check an .ISO image is an MD5 checksum, depending on where it came from. If it was downloaded, most sites have the MD5 available to compare against. You run md5 or md5sum against what you downloaded and compare the results and if it is different, there's corruption.

This entirely depends on where you got the .ISO file, though, and the availability of the MD5 checksum "known good" versus the one you have. Or if you have file integrity checkers (part of intruder detection) they should know if something changed in a file. Again, depends on how you had things set up.

If you have good backups, compare your first version of the ISO's MD5 with the current MD5 to see if things changed.