Net Stat shows services on the machine actually being used. Again look for weird host names and ports. Log files can tell if people have been connecting to machines, log ins and log in failures. Look for suspicious account names. Preventing Attacks Security vulnerability web sites are useful.
Patches are important.
Some platforms allow for automatic updating. Also important to keep in touch with security advisors for the various operating systems. Effective password management is also necessary. With SSH all communication is encrypted. There will be a policy mandating the use of Secure Shell.