How do I know the difference between being sad and depressed? Because I usually feel bouts of sadness, but it's becoming more often that I'm sad rather than happy. Is that an indication?

Being sad is one of the emotions. It may just pass after a couple of hours or a few days. You may be sad due to some factors but after some time, you will feel better. Depression is a feeling that may recur any time due to no reason at all. You just can't explain why you are feeling so low and you don't seem to know what to do. This is a case that has to be dealt with seriously because it might lead to severe anxiety and depression which is harder to treat.

If you have felt low mood (sadness) and / or lack of interest and pleasure in things for more than 2 weeks I would suggest speaking to a professional to see if it might be depression