By verifying with other sources.
Wouldn't it be much easier and simpler and more sane and logical and clearer and more accurate to simply read the original source rather than rely on books about books on books and South Park episodes?
That's what sets Calvin's Institutes of the Christian Religion apart from a lot of theology books -- Calvin based every word and concept and thought and principle on the words of God.
And if you doubt that, you can read them for yourself and tell us where he strayed from Scripture. (I don't think South Park has done any shows on Calvin yet so you may be forced to actually read him.)
So again, I believe Calvin is correct because I've read the Bible and I've read the Institutes and they align perfectly with the Bible.
How do you know the "other sources" you've read are correct?