Can I take legal actions if I detect some business is using our theme without license?Would Envato help us in this issue?

I am working on a theme and decided to sell it at Themeforest.I know almost all sorts of premium themes and plugins are available online,and businesses are using those for free.In most cases,theme or plugin author don’t know who is using their themes without license.
But if I can detect any business using our theme without buying license,can I take legal actions against them?Would Envato help us in this matter.
Websites that share nulled themes are real threat to theme authors who work for many hours to build a theme.What does themeforest do to stop theme sharing?
Thank you

Themeforest does nothing basically. That is author’s responsibility to take down those themes from the hosted servers by sending a DMCA take down notice to the hosting company.

Just wondering, how are you going to “detect” them?