You can check specific hosts to determine if a certificate has been issued to them.
The following command will return an exit status of '0' (true) if the host has a certificate, or '1' (false) if it doesn't.
# Where $HOSTNAME is either a variable to the host name or IP,
# or is a status hostname or IP
step ssh check-host $HOSTNAME ; echo $?
For more information on `step ssh` commands, see our command reference here.