By his faithfulness he was proved to be a prophet may be rendered “Because Samuel served the Lord faithfully, the people accepted him as a true prophet [or, a prophet of the Lord].”

And by his words he became known as a trustworthy seer is literally “and he was known in his words as one faithful in vision.” This could mean “People knew by what he said that his insight could be relied on.” Something like this might be better than using a word like seer, which is not found in the Greek anyway. See 1 Sam 3.19-20.

An alternative model for this verse is:

• Because Samuel served the Lord faithfully, the people accepted him as a true prophet. They knew by what he said [or, his words] that he truly spoke for the Lord.

Quoted with permission from Bullard, Roger A. and Hatton, Howard A. A Handbook on Sirach. (UBS Helps for Translators). New York: UBS, 2008. For this and other handbooks for translators see here.