Java: How to easily check if a URL was already shortened?

You could do a request to the URL, look if you get redirected and if so, assume it's a shortening service. For this you'd have to read the HTTP status codes.

On the other hand, you could whitelist some URL shortening services (t.co, bit.ly, and so on) and assume all links to those domains are shortened.

Drawback of the first method is that it isn't certain, some sites use redirects internally. The drawback of the second method is that you'd have to keep adding shortening services, although only a few are used widely.

One signal may be to request the URL and see if it results in a redirect to another domain. However, without a good definition of what "shortened" means, there is no generic way.