WSN Knowledge Base includes facilities to check for dead links, check for duplicate links, check for link partners who aren't reciprocating, and check for sites with undesirable content. These options are in the left side admin panel menu when you have the link checkers switch activated.

The checkers list suspects and prompt you to decide what to do with them. An explanation of what you can do to suspects is here

The duplicate checker simply finds instances where you have multiple copies of the same link.

The reciprocal checker searches for your directory url, site url (as listed at Admin -> Settings -> System), an in.php link to your directory, or whatever the "reciprocated with" field contains.

The dead link checker simply determines whether the site is accessible.

The content checker allows you to specify undesired phrases which, if found on the site, will cause it to be flagged for you.

Also see the troubleshooting article for link checkers.