There has to be some type of 'start', even the most shy man will look. Your mail carrier is an easy approach because you can ask any question relating to his job. You can say you were expecting a pkg, and ask how a large pkg would be delivered (if he asks about it later, say it came UPS), say your brother (sister, mother, whoever) wants a job at the post office and how should they go about it. Once you have that first conversation, you will be able to get a vibe on whether he likes you at all. If he's really shy, you may have to keep saying hello, and initiating the conversation. If he doesn't start initiating after the 3rd or 4th time, then it could be he's not interested.
As far as the computer lab, you won't know anything unless they look up, you hold the eye contact the first couple of times, the second or third time, you'll want to as a smile. If they just never look up, you can always act like you know nothing about computers and ask for help doing something on the computer "I see you at the computer all the time, maybe you can show me how to....." After that, you have an excuse to say hi to them when you see them
If they don't start initiating the hello's when they see you, again, you can take it that they are not interested.
Keep in mind that just because a man won't come on to you, doesn't mean he's not attracted, but he could be in a relationship, he could be gay, he could have just gotten out of a bad relationship, so don't take it personal, keep your options open.
I hope this helps. Let me know if you want to talk more