I'm on 10.15.2 Catalina, and my fans have currently been running steady for hours. I opened Activity Monitor, am viewing the CPU tab, and sorting by % CPU. Safari is at 102.6 as I type this.
Activity Monitor used to break down Safari by the open tabs, so you could easily identify the worst offender. Unfortunately in Catalina I just see "Safari".
How else can I determine which tab(s) is/are the main culprit?