If the system supports the "finger" feature (and many don't), you can try any number of guesses or permutations. If you succeed, you can find out lots of information about the person: their telephone number, when they last logged on, when they last read their mail, what department they work in, etc. Many systems allow you to leave a file called ".plan"—note the initial dot—that contains further contact information.
Good guesses for names:
o last name (bach),
o last name with one or more initials (pbach, pdqbach),
o three (or more!) initials (pdqb)
o nicknames, cute handles, etc. (fluffy,aragorn)
o work ID numbers (bach2378@bigblue.com)
Your last resort is a search program called "netfind". It lets you find a machine or person by keyword. If you know your target is at USND, you can try the keywords PDQ, USND, EDU and find pdq@hoople.usnd.edu. Note that you often have to guess the "domain", but this should not be hard. You can also search with PDQ, "University of Southern North Dakota", EDU, if USND is not sufficiently obvious. City and state names work, too. Try it.
The main short comings of "netfind" are:
(1) it often fails if the target computer does not support the "finger" command; and