How to stop a YouTube process running in Chrome? I have many tabs open in Chrome, and when I reboot it starts opening all the previous tabs, and when they contain YouTube videos they all start playing.

How can I find which tabs are running YouTube, so that I can pause them?

In the process list, there is no trace of YouTube, since it is running inside the browser, and only the tabs related processes are visible.

After reboot, it can happen that I have a YouTube process running and no corresponding tab from which to view it (I can just hear the audio).
You can open Task Manager, under Window on the Chrome.app menu bar, and see a list of all running processes inside Google Chrome.

You can see Task Name, Memory, CPU, Network and Process ID for each task running at the moment.

At the bottom of the Task Manager Window you can end the Task you selected by pressing End Process. 

[EDIT]

If you are on Windows the Task Manager window is accessible via the Shift + Esc shortcut.