Generally hearsay from another person is inadmissible, except in rare cases such as confirming that a missing husband has died (see Agunah).
A Beit Din may accept testimony only from a witness who speaks directly to the judges, not from a written deposition.
(However, the testimony of one witness can require a defendant to swear to his innocence or else pay the debt alleged against him.)
In monetary law two witnesses may absolutely require someone to pay a debt or absolve them from that obligation.
In contrast, in capital cases the judges threaten the witnesses by warning of the consequences of perjury (source: Mishnah Sanhedrin chapter 4), and they ask many questions and will invalidate testimony even for minor inconsistencies, even if the contradiction seems substantively irrelevant to the case at hand.
Testimony of a deaf, mentally incompetent or young person (before Bar Mitzvah) is excluded.
The Talmud, in the third chapter of Sanhedrin, delineates the rules governing who may provide written or oral testimony.
The Mishnah (Sanhedrin 24b) states: "The following people are disqualified: a gambler with dice, a lender who collects interest, a chaser of doves, and a merchant who profits from produce of Shemittah."
The Talmud explains that each of these four activities falls within an expanded definition of theft because people who violate Torah laws or social norms in pursuit of money cannot be trusted to tell the truth.