 Galatians, the fifth chapter, tells us what is the fruit of the Holy Spirit. Sacrificial love, peace, patience, joy, etc. And that's how you know a person to be from the Lord. In fact, as I think in the Didache, the Teaching of the Twelve Apostles from the first century, a very interesting book you can find on the web, the Teaching of the Twelve Apostles, or the Didache, D-I-D-A-C-H-E.