How does an author of a theme template knows that I reused that template?

for example I purchase a theme template with a regular license and then I sell the end product to my client but what if I then don’t delete that template on my computer and then I use that template for another job?
How can the author know that I reused it without purchasing another license?

There are several ways that this could be possible but obviously discussing these or encouraging people to try and circumnavigate it is not something for the forums