What command is used in Kerberos to determine if a ticket is valid?

The most appropriate command to determine if a Kerberos ticket is valid is klist. This command displays the content of the credentials cache, which includes information about the tickets that are currently held by the user. When you run klist, you can see the principal names, the issue times, and the expiration times of the tickets, which helps in verifying their validity.

On the other hand, kinit is a command used to authenticate a user and obtain a ticket from the Kerberos key distribution center. While it does play a role in ticket management, it is not used for checking the validity of an existing ticket. Instead, its primary function is to obtain a new ticket for the user, rather than assess the status of any current tickets. Thus, klist is the command designed specifically for reviewing the validity of tickets already acquired.