Filesystem checking has nothing to do with scanning for "bad" sectors. "bad" sectors in this context usually means sectors that can't be read reliably or can't be written reliably. Although a filesystem check can certainly stumble upon a bad sector since it attempts to read (and maybe write) blocks all over the filesystem.

What a filesystem check actually does is really up to each individual filesystem and up to the implementor of the filesystem checking utility. For some types of filesystems (e.g. nfs, tmpfs), the concept is meaningless, for others is is redundant because the filesystem continuously checks itself in the course of normal operation, and for others it is vital that to check the filesystem once in a while.

In general filesystem checking utilities are designed to check for corruption, inconsistencies, and violated invariants in the filesystem's data structures. If you want more detail than that, then you will have to ask about a specific type of filesystem.

Filesystem checking has nothing to do with scanning for "bad" sectors. "bad" sectors in this context usually means sectors that can't be read reliably or can't be written reliably. Although a filesystem check can certainly stumble upon a bad sector since it attempts to read (and maybe write) blocks all over the filesystem.

What a filesystem check actually does is really up to each individual filesystem and up to the implementer of the filesystem checking utility. For some types of filesystems (for example, NFS and tmpfs), the concept is meaningless, for others it is redundant, because the filesystem continuously checks itself in the course of normal operation, and for others it is vital that to check the filesystem once in a while.

In general, filesystem checking utilities are designed to check for corruption, inconsistencies, and violated invariants in the filesystem's data structures. If you want more detail than that, then you will have to ask about a specific type of filesystem.