Under your task manager, you can sort your programs by showing which use the most CPU usage from there, when it jumps you can see what's making this jump, if there is a particular program using a strangly high amount of processing power, then you know somethings up, but if its just all of your programs as a collective racking up 40% then you know that you've just got alot going, on,
see image in link to see the cpu usage section of W8 Task Manager if you dont already know how to do so.
http://cdn3.pcadvisor.co.uk/cmsdata/features/3422276/Win8-Task-Manager-2.jpg