I am upgrading Sitecore 8.0 to 8.1. In Sitecore 8.1
Sitecore.xDB.base key is required.
So how can i check that
Sitecore.xDB.base key is exists in license file?
Here's another way to check if your license does have the XDB enabled or not.
Steps are login to your content editor and then click on the RED top left side hamburger menu as shown in below image.
Then click on license
on opening model window scroll down all the way below where you will see entry named like sitecore.xdb.base and date. as show below in image.
If you do not see entry with XDB that means your license is not XDB enabled
Hope this helps.