The animal is believed to have been with his 59-year-old owner when she was found hanging from the ceiling of her Paris flat. Police believe it was suicide but her family demanded a murder investigation.
During a preliminary hearing the pet was led into the witness box by a vet to see how it reacted to a suspect.
It is said to have "barked furiously".
The aim was to decide if there was sufficient evidence to launch a full murder inquiry and a judge is yet to reach a decision.
French judge Thomas Cassuto praising the animal for his "exemplary behaviour and invaluable assistance".