seeing the Word at work...

1 Thessalonians 2:13 And we also thank God constantly for this, that when you received the word of God, which you heard from us, you accepted it not as the word of men but as what it really is, the word of God, which is at work in you believers. 

How do we know that the Word of God is the Word of God, and not just the word of men?  His Word has the power to transform lives.  That is what Paul observes here. From what he had seen and heard, these people responded with changed lives.   The question becomes: Have I seen His Word work in me?"