If I want to clearly judge whether a patent agency is formal or not, what can I ask them to show that they are a formal agency?

you can sign a confidentiality agreement with the agency, which is beneficial to both parties. Regular agency companies have professional ethics, and agents have the obligation to keep the information of customers confidential. If you are still not at ease, you can also apply in person, but it will be a little more troublesome, but it doesn't matter, so there will never be a problem of leakage.