I designed a trademark from which the client resigned, but after some time, without my knowledge and consent, he started using this project, which has been going on for about eight years. I am demanding 20 thousand PLN for the design of the mark, but the company that uses this mark questions my claims. Who and how can judge that my claims are right and assess the value of this trademark.

As to whether the claims are justified, a patent attorney or a law office dealing with proprietary copyrights should have an opinion. I can help you with the appraisal of your trademark. However, you must remember that if you want to relate the value of the claim to the value of the trademark, you will have to show a cause-and-effect relationship between the value of the trademark and the project (which is not obvious at all).