I read that Wu Zetian had a habit of executing officials who were accused of wrongdoing. What was her procedure of determining the guilt of the individual? Did it simply take a false accusation from someone holding a grudge to bring down another person?

If I was an honest official serving in her court, and someone holding a grudge against me falsely accused me of plotting to rebel against her, would I most likely be executed?

I'm curious how an honest official like Di Renjie was able to survive for so long in her court. Surely there must have been someone who was envious of him, and tried to falsely accuse him of some wrongdoing.

Normally， the process of determining innocence/guilt was handled by the Justice Department (刑部）within the chinese bureaucracy to investigate whether a person is innocent or guilty of crime.

The Emperor will have to rely on good honest official to provide advice and information. He will then need to use his logical wit or power to investigate about the truth of such accusation.

However, as already been mentioned, the Chinese Emperor is above the law (absolute power). He/she had absolute power to determine whether a person can live or not. That meant to say, if the Emperor wanted an official to die, he had no choice but to die (君要臣死，臣不得不死）.