You can find if encryption option is provided on a remote telnet server using this tool.
Determines whether the encryption option is supported on a remote telnet server. Some systems (including FreeBSD and the krb5 telnetd available in many Linux distributions) implement this option incorrectly, leading to a remote root vulnerability. This script currently only tests whether encryption is supported, not for that particular vulnerability.
You can download the latest patches and follow the required steps from the references part not to be vulnerable.