How could I tell if I have OCD or depression?

See your doctor. Schedule an appointment to talk with your doctor about your concerns. He/she will be able to evaluate and/or refer you to a psychologist or psychiatrist for evaluation to determine any mental health diagnoses present. They will also be able to discuss treatment options with you to help you determine what is best for you.
Different symptoms. One can have both OCD and depression, but they are quite different disorders with very different symptoms. Ocd has to do with obsessions and compulsions, for example, the need to wash one's hands repeatedly or to do things "evenly."--think monk, if you ever saw that series. Depression has to do with mood and related symptoms like poor sleep, low energy, suicidal thoughts, that persist.
Anxiety or sadness? Ocd has an anxiety feel to it (it falls under the anxiety disorders) & has elements of obsession & compulsion; depression has more a feeling of loss & sadness to it... To make it a little more difficult, depression can involve at times some anxiety/agitation. Of course, whichever one it is, a therapist would be worth seeing to answer your question in a more personalized way & to help. Good luck!
Ocd. Intrusive thoughts and rituals to reduce anxiety, is common in ocd. Depression shows neurovegetative symptoms and sadness.
See U PCP . He will evaluate u and according to his finding will refer u for treatment .