Welcome to Healing Well, Koukla!
In response to your question -- how does one know if one is depressed -- well, darlin', it's different for everybody. I, myself, noticed that the "bluesy" feeling I'd been having seemed to last for weeks...for no reason, really, and with lots of different effects. Like you, I had no motivation (just getting out of bed seemed like a battle!), I felt generally apathetic (although there'd be times when I'd start sobbing out of nowhere), and I just didn't feel like going out or being with my friends. The hopelessness got so bad I decided it was time to help myself...and so I made an appointment with a psychologist. Since then, I've seen her twice, I've been to a psychiatrist for medication, and now I'm signed up with a campus therapist -- woo hoo for college -- whom I see once a week.
As for suggestions, I'm an advocate for making an appointment with a psychologist or a counselor. They really helped me to understand what my feelings were, and they opened up avenues of treatment I wouldn't have had access to before. If you don't want to take that step, you could talk to someone you trust -- a friend, a family member, etc. -- and let them know how you're feeling and that you're looking for suggestions.
Oh, and I just finished a wonderful book called "Overcoming Depression" by Paul Gilbert...so maybe you could look it up :)
Keep in touch and let us know how things turn out...we're here to support and encourage you!