Proving a wrongful termination claim is incredibly difficult without having the right evidence. The reason is that Illinois is an at-will employment state, meaning your employer can fire you for any reason or none at all.
Because of that rule, the only way to prove wrongful termination is if you have a contract or if you have documented proof of discrimination.
A contract is a legally binding agreement between you and the employer. If your contract contains a clause about termination, then your employer must abide by that clause. If not, you have the right to make a legal claim. But if your contract does not state anything about the length of your employment or how to end the contract, then you may not have a strong case. You have to prove your employer violated the terms of the contract.
Discrimination is the only other reason why you would have a valid wrongful death claim. Any type of discrimination is illegal, and an employer cannot fire an employee due to it. Still, proving discrimination is tricky. You would want to have evidence showing an ongoing issue and past times your employer discriminated against you. Essentially, you want to show that your employer treated you differently than your coworkers and that the motivation was because of a bias against you. It is not easy to prove such a case because your employer knows this is illegal and will try to cover up actions that could help you prove your case.
Still, if you know your employer wrongfully terminated you, it is well worth looking into filing a case and seeing if you can hold your employer liable for breaking a contract or discrimination.