They will do as good a check as they can, but if you have a fraudulent document, it often takes time if you really want to try to get to the root of the problem. Sometimes visa officers will have to go over to the institution themselves. They certainly will do a visual check of all the documents as best they can, but they do not have time to do as thorough a check as we should be doing, especially with the very high levels of fraudulent documents that are submitted at a lot of our posts in developing countries. Some of our largest posts get a lot of fraudulent documents.
On February 28th, 2012. See this statement in context.