How to Know if Your Supplements Work

How do you know if your supplements work? Does anybody know if their supplements work? How can you possibly tell? Because you can't tell experientially and there's no science to back it. I don't even know if the magnesium that I bought because I got some special powder to kind that you put in water is supposed to be better. Should I keep taking it? Maybe creatine? Yeah. Well at least creatine you could see if it's working, couldn't you? You'd probably see a muscle difference in three weeks. Am I right? Three weeks and you'd know if it worked. But how in the world will I ever know if magnesium is working? I don't