How can they figure out that a paid font is being used for free?

Question

Fonts

Good afternoon. Let me just say that stealing is bad. I just saw a lot of sites with expensive fonts that just can't spend that kind of money.
Purely hypothetically, for example, I just download paid fonts from some site, place them on my own. How will copyright holders know that I stole their font? Only if they accidentally visit my site or someone files a complaint?