[jj88]

How can you tell if you're depressed, or just sad, or if you're normal?

[Tauren]

That's really a question for a psychiatrist, but there are a lot of "screening" quizzes. Here's one:
Depression Screening | Mental Health America

[Fizzyo]

Hi, sorry life is tough for you. There are some good depression screening quizzes on this site, click on 'Quizzes' at the top of the page. The best way to tell is to speak to a doctor, but my idea of the difference is that if you're sad, there is often something you know you're sad about and over a few days or weeks the sadness will reduce, you may also find you can distract yourself to feel more like your normal self at times, eg with supportive friends.
Depression lasts longer and may be more overwhelming, (though bereavement sadness is often totally overwhelming and is normal) if your sadness has no reason you are aware of, or has lasted for longer than a few weeks or feels unrelenting or as if it would never end, or you have had thoughts of harming yourself in any way, it would be a good idea to discuss it with a doctor sooner rather than later for an expert assessment and advice about what might help you. Best wishes and good luck as you look for a way forward. Don't suffer in silence. I hope you find the support you need.