There's no obvious way I can think of using a search engine or similar, google has a search for sites that link to certain pages, but that's it.
If I was approaching this seriously, I'd knock a program up that crawled all the pages on the sites and noted how many links there were in the source (count the HTML anchor tags.) I'd run the program, make a note of it and store the state of the current pages, then run it again when the link had been added. It'd then hopefully alert me of an extra appearance of an anchor tag, and a quick compare of the page in question with its previous version should reveal the extra link.
Not foolproof, but it's the easiest thing I can think of quickly that might work.