If you think you have a deficiency, explain why to a doctor. A blood test to know for sure is simple. A doctor will know what kind of supplementation would best serve you, and there may be an underlying reason that can be treated to fix it.
I didn't say "no one should take supplements ever," I said most people who take supplements are doing so unnecessarily, and you should do so under the supervision of a physician.
I'd like to note in my top level comment I was referring to medically unnecessary vitamin and mineral supplementation. Protein powder is food and is not part of that. It's 100% necessary for serious lifters, but it's definitely also overused by people who are not serious lifters.