How do you know that the blacksmith is a honest man?

Longfellow paints a very flattering portrait of the village blacksmith. This is a man who seems to have no negative character traits whatsoever. He's incredibly hard-working, he regularly goes to church, he's immensely proud of his little daughter when she sings in the village choir, and he still cries over his dear, departed wife. What's not to like?
What's more, the village blacksmith is an honest man, as Longfellow makes clear in the second stanza:

His hair is crisp, and black, and long, His face is like the tan; His brow is wet with honest sweat, He earns whate'er he can, And looks the whole world in the face, For he owes not any man. (Emphasis added)

This is a hard-working man who toils each day at an honest trade. Moreover, he's fiercely independent and self-reliant; he owes nothing to anyone. But we don't simply have to take the speaker's word for it. We can reasonably infer that if the village blacksmith weren't an honest man, then it's highly unlikely that much work would ever come his way. Reputation is everything in such small villages, and a reputation for dishonesty would almost certainly put the blacksmith out of business.