If you have the money for a blood test, that will give you a good idea of what you might be missing. A DNA test can also direct you in a similar way. I just found out I might have an issue with converting b12 at the usual rate. This was found when i used my 23andMe raw output in some 3rd party websites.
If you really want to do things right, I think you should also pay attention to the bio availability of supplements. My understanding is that not all vitamins are made equal. This can lead to headlines dismissing the usefulness of supplementation altogether.
I try to focus on whole food supplementation when I need it. I personally take Athletic Greens at 1.5x the recommended dose, along with D3 as 10,000 IU (much higher than the base recommended dose because I live in the UK) and calcium (because I can't eat dairy).