If we can adequately define much less obtain a balanced diet. I am dubious. No, not doobious - dubious. ;-)
I take a multi-vitamin/mineral/whatever pill containing no mega-doses. That's so I get at least some of all that good stuff we're supposed to get. I take additional vitamin D because I don't believe we get what we need from either sunlight or diet.
We don’t need to define a balanced diet, nor do I think it is possible.
You can tell for yourself whether you are eating properly or not. First of all, your body, if you know how to listen to it, gives signals that direct your food choices.
Obviously, someone who eats only celery every day is not consuming a balanced diet. Someone who eats a variety of foods every day is.