Rambam, Hilchos Eidus, Chapter 11: 

Chapter 11: Further Rasha disqualifications

Halacha 1: An ignoramus

• An individual who does not study Torah, Scripture, or the Mishneh, and does not socially participate in the religious community, is automatically considered a Rasha, and his testimony is rabbinically invalid, as certainly such an individual transgresses many of the laws.

Halacha 2: The validation process for an ignoramus

• Accordingly, ignoramuses should not be designated as witnesses, and one should not accept their testimony, until it is verified that they are observant of the mitzvot’s, perform acts of kindness, are upright people, and are socially involved with the community.
• If the above is verified regarding the individual, then he is valid as a witness even though he is unlearned and does not study Scripture or Mishneh.

Halacha 3: The difference between an ignoramus and a Torah scholar

• Based on the above it is understood, that a Torah scholar is assumed to be a valid witness until proven otherwise, while an ignoramus is assumed to be an invalid witness until proven otherwise.

Halacha 4: Not to accept testimony of an ignoramus until he is validated

• Whoever accepts testimony from an ignoramus prior to verification of his validity, is considered a fool and he will be punished for doing so, as he causes the money of the Jewish people to be lost due to the testimony of Reshaim.
• If two witnesses testify regarding an individual that he is righteous and upright, then he is valid as a witness.

Halacha 5: The invalidation of shameless individuals

• Shameless individuals are rabbinically invalid as witnesses. The following people are considered shameless:
  • Individuals who eat shamelessly in public while walking.
  • People who walk around unclothed in the marketplace when they are dealing with a undignified task.
  • People who publicly live off the charity of Gentiles without any shame, even though they have the opportunity to do so in a modest setting.
• All the above people are considered invalid, being that they have no shame, and are considered like a dog, and have no issue with giving false testimony.

Halacha 6: The difference between biblically invalid witnesses versus rabbinically invalid witnesses

• The following is the difference between a biblical versus rabbinical invalid witness:
• Biblically invalid witnesses: A biblically invalid witness is considered automatically invalid even prior to him being proclaimed as an invalid witness.
• Rabbinically invalid witnesses: A rabbinically invalid witness is not considered automatically invalid. His invalidation only begins after it is proclaimed in the synagogues and houses of study. Therefore, any testimony that he gave prior to this announcement, is valid.

Halacha 7: Believing a single witness by matters of prohibition [i.e. Eid Echad Neman Beissurin]

• A single witness is valid for testimony regarding matters of prohibition.
• Believing him regarding Kashrus: This applies even if he is considered invalid for other testimony, such as due to being a Rasha, as even a Rasha who slaughters an animal, the animal remains kosher, and he is believed to say that he slaughtered it properly.
• One who is a Chashud: The above is with exception to an individual who is suspected of transgressing a certain matter, in which case he is not believed to testify regarding his own item, however he is believed to testify regarding the item of someone else.

Halacha 8: One who is suspected of transgression

• Accordingly, one who is suspected of a certain transgression may judge and testify for other people regarding it.
• The logic behind this allowance is because a person does not sin in order to benefit other people, as the fear of transgression is found even upon the wicked, even though they do not fear causing others monetary loss.
• Testifying on Maaser: For example, even an ignoramus is believed to say that a product has been properly tithed.
• Testifying on non-Bechor meat: Likewise, a person who is suspected of selling meat of the firstborn animal, is believed to state regarding a piece of meat that another person is selling that it is not from the firstborn.

Halacha 9: Testimony from and on behalf of kings of Israel

• One may not testify regarding a king of Israel. Likewise, the kings of Israel are considered invalid witnesses being that they are ruthless and controlling, and do not subdue themselves to the judgment of the court.
• A high priest: The testimony of a high priest is valid, and he may testify on behalf of the king or the president of the court. Likewise, one may testify regarding the high priest.

Halacha 10: Heretics and informants

• Invalidation of heretics as witnesses: Heretics and informers are automatically invalid for testimony, and did not even need to be listed by the sages amongst the list of invalid witnesses being that they are worse than Gentiles.
• The rule by Gentiles is that one may not help save their lives but also may not actively cause their death.
• Reward for Gentiles: Likewise, the pious amongst the Gentiles have a portion of the world to come.
• Punishment for heretics and informants: However, the heretics and informers are to be actively brought to their death, and they do not have a portion of the world to come.