Especially where the whistleblower has tried to be anonymous, it may be hard to prove the employer has knowledge of the protected activity. Still, some courts will use inferences to deduce who the employer may have suspected. If you were among a select few who had the necessary information and the courage to speak up, that might be enough for a judge to infer employer knowledge. Sometimes, the employer's investigation or interrogation of an employee can reveal the employer has knowledge of the protected activity. To avoid this problem, some sophisticated whistleblowers will announce their protected activities. If they disclose copies of evidence to an agency, they can send a copy to the employer by certified mail. Certified mail has the advantage of creating a document that shows the date the employer received the item. If the retaliation occurs shortly after the whistleblowing (say within six months, or sometimes longer), then the timing alone may persuade a judge that the employer's true motive was because of the whistleblowing. However, if you are blowing the whistle on criminal activity, it would be best to get experienced legal advice before disclosing your protected activities. Disclosure can have the effect of telling the crooks that the cops are coming after them – which could be a form of obstruction of justice.