https://qz.com/1085171/how-to-tell-if-your-computer-is-secretly-mining-cryptocurrency-and-what-to-do-about-it/

Your computer could be secretly mining cryptocurrency—piggybacking on your computer’s processing power to confirm transactions and generate new and potentially lucrative coins—and you wouldn’t even be profiting from it. Incidents of malware containing crypto-mining tools have surged six-fold this year, according to IBM Managed Security Services. Here’s how to find out if you’ve been unwittingly committing your computing power to enrich someone else.

Open a resource monitor on your computer to check if CPU usage is abnormally high. On a Mac that’s Activity Monitor, and on Windows it’s Task Manager.