If you want to run fsck on a loopback filesystem (n_ormally a virtual machine disk image like xen for ex._) you just have to tell its proper filesystem type when you run fsck:

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fsck.ext3 /path/to/loopbackfile.ext3

for ext3 or:

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fsck.reiserfs /path/to/loopbackfile.reiserfs

for reiserfs, etc.