If you are a lay person, as I am, you can still make judgements symptomatically. If you have good reason to believe you are probably deficient, you can conservatively experiment with taking supplements and see if it helps. If your status improves, adjust doses downward as needed.
Keep a journal. Keep a journal. Keep a journal.
If you can't manage it yourself in that manner, yes, seek testing and so forth.
Don't ever take supplements of any kind for any reason if you don't have reason to believe you in particular actually need the supplement. Period.