You don't - ergo good faith.

You can be an Oracle and audit your customers and develop that adversarial relationship. The idea is that that sort of thing makes you rot in the long run.

I am sure everyone making shareware in the early 1990's would have loved to spy on people to know how many used their software for free (and have a way to spam those users to try to sell more licenses), but they couldn't and just did without that.