Answer:
Unlike what the other person said, you don't need a medical diagnoses to know if you are depressed.
You need a medical diagnoses to get medication for depression and, if it's so bad that you can't work / go to school etc. then you need that for disability benefits.
However, usually you can tell if you are depressed / have depression by the following (mind you this is a list I pulled from my own therapist and my own symptoms. I was professionally diagnosed in October of 2020 with Major Depressive Disorder, or MDD, however I *knew* I was depressed LOOONG before then):
1. Trouble Sleeping / Sleeping more than usual
2. Lack of concentration
3. Short-Term Memory Loss
4. Anger / Irritability
5. Withdrawal from others
6. Changes in personal appearance (This isn't hand in hand, but it does go along w/ depression)
7. Thoughts of offing yourself / self harm
8. Weight changes (either gaining weight a lot, or losing a lot of weight, even if your diet doesn't change)
9. Abandoning your hobbies
Mind you, this is only a small list.
If you are seriously concerned about your mental health there are multiple online and in-person resources you can use to talk to someone about this!