So there's that really cute guy or girl that keeps hanging around at your desk at the office (or at school, or at the park) and you're wondering if they're interested in you, or if they're just killing time. This is a fine line to balance along and your best bet is to really observe their actions and reactions to you. Add that to your gut instinct and you'll get a pretty vivid picture. Of course, you can always break down and do the obvious thing--ask them if they're interested in you. Here are some ways to try and figure out if someone likes you if you're too shy to come out and ask.
Notice that this person is always hanging around with you lately and calling, writing you notes or emailing you just to say "hi." If a person fancies you, they won't be able to stay away from you.
Notice if this person is coming to you for advice and really listening and taking your responses to heart. If you go on a date with someone else, see if this person seems bummed out instead of glad for you.
Pay attention to when you go out in a group. See if this person is always the last to leave you and is always by your side, laughing at your jokes.
Note if this person seems to melt or get nervous if you ask them out and gets flustered/blushes/responds enthusiastically if you hug or kiss them goodbye.
See if close friends of yours have noticed and commented that this person seems to really fancy you. Sometimes your friends see things that you are blind to seeing.