As the TURN server speaks the STUN protocol, a simple way to test it is with a STUN client.
Example 10.8. Installing the stun client utility
stun
$ sudo apt-get install stun
Example 10.9. Using the stun client utility
$ stun turn-server.example.org STUN client version 0.96 Primary: Firewall Return value is 0x00000b