Often we are looking into issues for clients and need to raise support ticket with Microsoft.
One of the first things asked by Microsoft is to know which version of rollup and CRM we are on.
In 2011 , it was pretty easy with File menu.
But in CRM 2013 and CRM 2015 also there is a quick trick to check the version:
Just navigate to this URL(this is CRM 2015 online):
You will be able to see a screen with version(See highlighted in screenshot below):
Hope it helps!