How do I know if the supplements I'm taking are doing me any good?

How do you feel? If you have a good, balanced diet with lots of fruits/veggies etc..And stay active, u may not need any/much supplement. You should find out what you need and supplement that need. If taking a reasonable amount of supplement making you feel better and not causing side effect, then it is ok. Excess of anything is not good. Regular follow-up with doc also a good thing..Stay well... Good luck.
Many things. Vitamins are not a regulated item for the fda. Studies show that over 50% of the vitamins sold make no changes. For better vitamins, use physicain grade supplements (ex douglas labs, metagenetics) your levels will improve by lab work, you will feel a step up within a week & test them at home. Get regular vinegar, put your pills in their, see how quickly they dissolve, if <15minutes, they are ok.