So to summarize, as of now ACS does not log anything, you can check the hit count on the Access service to see whether the packet was received by the ACS, rest you can only capture it using a Sniffer or a simply observe logging on the radius server.
Note: The behaviour has changed in ACS 5.4 and ISE:
The proxy requests are logged in ACS view. However, The failure reasons are shown empty. In ISE, the failure reason is thrown as "Please review the logs on External server to determine the precise Failure reason.