A site similar to 12ft.io but is self hosted and works with websites that 12ft.io doesn’t work with.
How does it work?
It pretends to be GoogleBot (Google’s web crawler) and gets the same content that google will get. Google gets the whole page so that the content of the article can be indexed properly and this takes advantage of that.
Google literally has an official list of IP ranges for their crawlers, complete with an API that returns the current IP ranges that you can use to automate a check. Hardly a moving target, and even if it is, it doesn’t matter if you know exactly where the target is at all times.