If your employer did not know that you were trying to organize a union, then the employer will argue that the decision to fire you could not have been in retaliation for that union organizing. Many employers use this reasoning to escape liability. Especially where workers are trying to organize under the boss's radar, it may be hard to prove employer knowledge of the protected activity. Still, the NLRB will use inferences to deduce who the employer may have suspected. If you were among a select few who had 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 employer knowledge of the protected activity. To avoid this problem, some union organizers will announce their protected activities. For example, you could send the boss a letter (typically by certified mail) that says, "I plan to start talking with my coworkers about organizing a union." It could describe other protected concerted activities that you plan. This type of letter is called a "revealment" letter because it reveals who is organizing for the union. 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 organizing effort (say within six months, or sometimes longer), then the timing alone may persuade a judge that the employer's true motive was retaliation for your union organizing.