I get the feeling that you are relatively young. When I started in design, I thought my ideas would be stolen. Then I worked for 14 years and realized that even when you jam your greatest idea down someones throat, they get ignored.
a. Patent clerk. These people are way to busy with their job to go and try to sell some idea on the side. Plus, they would likely face termination for violating the privacy of their clients.
b. Naturally, people really like their ideas and think your ideas are dumb. Until you are making $300million on your idea, no one will pay any attention to you.
As for your other question, “how does a designer know how many items were sold?” - ultimately, they don’t. Hopefully, the company has some requirement to publicly disclose some data (SEC filings) that can be used to double check them. Legal action could probably result in some disclosure as well. Ultimately though, if the product sells well, the client will want to work with you again and won’t burn the bridge by trying to hide a few grand in royalties.