For me, I used to take Centrum (a popular multivitamin my dad had me take since I was 10 or so) daily for decades, I stopped several months ago when I ran out and was too lazy to get more. I felt no different. I would assume I'd feel a deficiency not taking it but I never did. I started having doubts, so I changed things up a bit.
What I take now as supplements, not all daily, but often:
- Protein supplements for exercise (whey isolate only with no flavor and minimal ingredients)
- Iodine, a drop a day, the only daily one I still use because I can feel when I don't
- Milk thistle for my liver - I like to eat a lot of toxic raw seafood like oysters and it seems to help
- Fuck loads of B-vitamins after I drink since alcohol burns B-vitamins fast af
For avoiding:
- Fluoride - I buy expensive water filters to get rid of fluoride. If you take care of your teeth, there is no need for it. It can only harm you.
- Tap Water - Similar to above because I live in SoCal and sewage is treated and dispensed as drinking water. The pharmaceuticals are not filtered well at city level, so I filter them myself. I used to take my male cat to the vet for urinary crystals (kidney stones for humans) multiple times/year, I started filtering it with good filters, never had a problem with him since. Thousands of dollars lost to him not being able to pee from drinking tap water, never has an issue with properly filtered water. Please do the same. I may gift you a filter if you need if you have cats and message me if you can't afford it.
If you willing to spend some money, you can get a blood test done that will tell you what your nutrient levels are like. Many of these will test for 25-35 nutrients at once. The results are sort of subjective, but the numbers can be reasonably compared to a second test, giving you a good idea of how dietary changes have improved your nutrient levels. Its also worth noting that what most tests list as the "normal range" is not necessarily the same as "healthy range." If something like 1 in 4 people are zinc deficient, you can expect the healthy range to be somewhat higher than the "normal range."
Alternatively, you can just check your symptoms against a list, try taking a recommended dose for a few weeks, then check if symptoms have dissipated. When taking zinc (assuming you are deficient) you might notice your mood improve within a day or two, but dry skin might take a couple weeks to improve.