Occasionally I hear of sites being penalised by Google and quickly dropping out of the SERPs. This can often be the result of spammy links being injected into the site’s template.
Often the links are hidden from everyone apart from the search engine spiders, so a quick normal visual check won’t reveal the problem.
In those cases a site search at Google for the domain name + a spammy term can show the infected articles.
The following bookmarklet will take the domain you are currently visiting and search Google for site:domain and add four common spam terms. Each Google search is opened in a new window giving you the original site plus four new tabs for each of the searches.