You can use the EFSINFO tool to find all encrypted files on a volume. This tool is found in the Windows Server 2003 Support Tools on your product CD. To use this tool to find encyrpted files, run the following from a command prompt:
efsinfo /s:c: | find ": Encrypted"
Why would you want to do this? Well for example, if you have a server used for a workgroup and you decide to promote it to a domain controller, you need to remove all encrypted files from it first otherwise they won't be accessible afterwards.