What is the easiest/fastest way to check if a Magento 2 site is on EE or CE?
Preferably via the admin if possible.
You can get this information for any Magento2 install from Magento's official endpoint, by opening this URL in your browser:
That will give data like:
You can detect if a website is running Magento Enterprise Edition by hitting website_base_url/giftcard/customer in your browser. If you get a 404 Page Not Found error then that means they are NOT running MEE (Magento Enterprise Edition). If you get redirected to a login page that means they ARE most likely running MEE.
More Info Here