Determine the Installed SharePoint Edition: Standard or Enterprise

Here is how to check whether standard or enterprise edition is installed in SharePoint 2013.

Go to Central Admin >> Upgrade and Migration >> Click on "Enable Enterprise Features"

This procedure stays same to check standard or enterprise SharePoint 2010 and SharePoint 2007:

• When the “Enterprise” is selected and grayed-out, it’s an Enterprise edition of SharePoint!

•  Here is the Standard edition screen: If, Standard option is enabled, then we can determine its a Standard edition.

SharePoint 2013 standard to enterprise upgrade:
You can use this page in SharePoint to upgrade standard to enterprise. But you can't downgrade SharePoint enterprise to standard.