Chapter 3 Authentication for FortiOS 5.0 : Authentication servers : RADIUS servers : Configuring the FortiGate unit to use a RADIUS server : Troubleshooting RADIUS
  
Troubleshooting RADIUS
To test the connection to the RADIUS server use the following command:
diagnose test authserver radius-direct <server_name or IP> <port number> <secret>
For the port number, enter -1 to use the default port. Otherwise enter the port number to check.
Additional RADIUS related troubleshooting is located at “Troubleshooting FSSO”