A WebmasterWorld thread has one link builder asking other link builders how they go about verifying sites are actually linking to their sites.
Link builders get others to link to their site. They do this by writing excellent content, providing linkable stuff, begging, asking and even paying for links. Once you get the link, you want to make sure it sticks - especially if you are paying.
I am pretty sure the more advanced link building companies build software to check sites on a routine schedule. They build spiders that sniff out pages where the link should reside. They aren't 100% foolproof but they help.
Other companies use other tools to aid in ensuring links are where they should be. Some use Yahoo Site Explorer, some use Google Webmaster Tools, some use advanced search queries.
While others simply do hand checks from a list of sites they check manually.
What do you do?
Forum discussion at WebmasterWorld.