The Resources tab of the System Monitor tool allows you to view the current CPU usage on the system.

To start the System Monitor tool, either select Applications → System Tools → System Monitor from the Activities menu, or type gnome-system-monitor at a shell prompt. Then click the Resources tab to view the system's CPU usage.

In the CPU History section, the System Monitor tool displays a graphical representation of the CPU usage history and shows the percentage of how much CPU is currently in use.