A customer can generate a token via the customer area for each license. You can then use that token to validate information about the license. You can also confirm if it's attached to a particular domain if you know what to lookup (without us actually giving you the full URL that it's attached to).
It can be accessed via JSON as well as the web interface. Note that there are limits to the number of times it can be hit per day.
All the details are discussed on that page.
(The link to that page is on the footer of every page, BTW.)