You can't. More often than not, mental illness goes hand in hand with unorthodox thinking that breaks the mold and causes unexpected progress.

We could even say that mental illness is defined by falling significantly out of alignment with the median mind of society. A mentally healthy person in our modern society might be seen as completely insane and unwell in a hunter gatherer tribe.

But we generally don't get to work out who was just mentally ill with no value and who was a value add until well after their death.

"Society honors its living conformists and its dead troublemakers."