How to detect if people remove your footerlink on a free custom style?

[Opserty]

You can't, you can easily view PHP source code (unless encoded with something like Ioncube or Zend encoder - however that would place requirements on the server which would need to have a decoder/parser installed on it), therefore any method you put in place could be easily mimicked or just removed completely.

You'd have to just keep track of who has used it and then check up on them.