General synonyms:
* Observer: Someone who sees or notices something.
* Spectator: Someone who watches an event.
* Bystander: Someone who is present at an event but not involved in it.
* Eyewitness: Someone who personally saw an event.
* Testifier: Someone who gives evidence in a court of law.
* Attest: To give evidence or confirmation.
* Verify: To confirm the truth of something.
* See: To perceive something with one's eyes.
* Observe: To watch something carefully.
More specific synonyms:
* Deponent: Someone who gives sworn testimony.
* Informant: Someone who provides information.
* Informant: Someone who provides information, often secretly.
* Stipulator: Someone who agrees to something.
* Affiant: Someone who makes a sworn statement.
Other related words:
* Evidence: Something that proves or disproves something.
* Testimony: A statement given in a court of law.
* Proof: Something that establishes the truth of something.
* Documentation: Written or recorded evidence.
The best synonym to use depends on the specific context.