There are two main mechanisms, or perhaps three. The first one is that of document verification by the Indian authorities, and these documents are sometimes fakes. The second tool, that is mostly used when the documentation is not verifiable or when the results are not clear, is DNA testing. In some situations, we visit the village where the family lives and we question neighbours in order to try and determine who really lives in a given dwelling.
On February 15th, 2011. See this statement in context.