To view a certificate, use the -L to list the contents. For example, for a user's certificate:

~]$ ssh-keygen -L -f ~/.ssh/id_rsa-cert.pub
/home/user1/.ssh/id_rsa-cert.pub:
        Type: ssh-rsa-cert-v01@openssh.com user certificate
        Public key: RSA-CERT 3c:9d:42:ed:65:b6:0f:18:bf:52:77:c6:02:0e:e5:86
        Signing CA: RSA b1:8e:0b:ce:fe:1b:67:59:f1:74:cd:32:af:5f:c6:e8
        Key ID: "user1"
        Serial: 0
        Valid: from 2015-05-27T00:09:16 to 2016-06-09T00:09:16
        Principals:
                user1
        Critical Options: (none)
        Extensions:
                permit-X11-forwarding
                permit-agent-forwarding
                permit-port-forwarding
                permit-pty
                permit-user-rc

To vew a host certificate:

~]# ssh-keygen -L -f /etc/ssh/ssh_host_rsa_key-cert.pub
/etc/ssh/ssh_host_rsa_key-cert.pub:
        Type: ssh-rsa-cert-v01@openssh.com host certificate
        Public key: RSA-CERT 1d:71:61:50:05:9b:ec:64:34:27:a5:cc:67:24:03:23
        Signing CA: RSA e4:d5:d1:4f:6b:fd:a2:e3:4e:5a:73:52:91:0b:b7:7a
        Key ID: "host_name"
        Serial: 0
        Valid: from 2015-05-26T17:19:01 to 2016-06-08T17:19:01
        Principals:
                host_name.example.com
        Critical Options: (none)
        Extensions: (none)