Many companies of all sizes require or at least strongly request that employees take exit surveys prior to leaving their last day of work. It helps them understand more about the people working at their firms, and the things they may be doing that would make an employee want to go elsewhere for employment. Even those fired for wrongdoing should take a survey like this, if only to make the termination official.
If your company requires a way to understand what former employees truly think about what it was like to work there, you need to formulate an exit survey. However, resist the need to do this part internally and instead go with a reliable provider of exit surveys to ensure the most relevant and appropriate questions are asked of these former employees. A provider has a greater understanding of how to word questions to get the most helpful responses.