The Greek that is often translated as “testify” in English has the option of terms in Luang with different shades of meaning.
For Acts 10:42, nala linniohora-matniohora (“give ear-eyes about”). “This term is used when the witness actually saw the event and testifies truthfully about what he has seen.”
Source: Kathy Taber in Notes on Translation 1/1999, p. 9-16.
See also testimony.