The validity of the key can be checked with the command

Step-by-step guide

Use the following command to check the validity of the key,
ssh-keygen -L -f ~/.ssh/cscs-key-cert.pub

Response : 

Type: ssh-ed25519-cert-v01@openssh.com user certificate
        Public key: ED25519-CERT SHA256:pF3znTpw2EyGkjeCLnhXqGjf0Ar0RXcomyVN+kxmZmI
        Signing CA: ED25519 SHA256:BwybyU6cNJBS7AX6BdwgJV2emGDV4lkKY4413WRGboY (using ssh-ed25519)
        Key ID: "username"
        Serial: 0
        Valid: from 2021-09-30T09:31:26 to 2021-10-02T09:31:26  
        Principals:
                username
        Critical Options: (none)
        Extensions:
                permit-X11-forwarding
                permit-agent-forwarding
                permit-port-forwarding
                permit-pty
                permit-user-rc