I had previously written an blog about this
and the method involved getting the url and copying out the guid from it, which is a bit slow and painful, particuarly on non default entities because there are two guids in the url and I keep forgetting which one I need.
Also someone informed the url is no longer displayed in CRM 2013!
Then a comment from Michael Blackburn gave me a much simplier solution, which I thought I should share because it’s certainly made my life a lot easier.
Press F12 to open the developer console, Click the script tab, and in the bottom right, next to the >> type in:
Here is video link, showing you how to get the guid in Chrome and IE in CRM 2013