Is it hard to prove retaliation in the workplace

This is a very complex question and will depend on the circumstances so there is no way to answer it without a lot of information. However, keep in mind that proving retaliation is often of limited value to an employee.  Your employer is not legally required to do anything about "retaliation."  You employer is required to do something about "unlawful retaliation."  If someone is suffering from unlawful retaliation, it will make a big difference whether you can prove it or not.  If it is just plain retaliation, proof only matters if your employer is willing to address the situation.