You must provide the judge with proof in the form of documents and testimony. Gathering this evidence requires depositions, expert witnesses such as accountants, interrogatories, witness interviews, subpoenaed records, and evidence presented following the Rules of Evidence and subject to the Executor's objections.
To be nominated to be the Executor of a Will imposes upon the person so appointed a fiduciary duty to adhere to the terms of the Will in conformity with California law. That duty can impose personality liability upon the Executor should he or she fail to perform as required.
State law varies, but courts generally focus on what is in the best interest of the beneficiaries. A court can hold the executor personally liable and award damages. If the executor's conduct is bad enough, the court can award punitive damages to punish the wrongdoer.
If an executor in California commits misconduct while handling the estate of a deceased person, the heirs and beneficiaries may be able to get their rightful assets back by filing a lawsuit against the executor.