Linkback and Webmention

A linkback is a method for Web authors to obtain notifications when other authors link to one of their documents. This enables authors to keep track of who is linking to, or referring to, their articles. The four methods (Refback, Trackback, Pingback and Webmention) differ in how they accomplish this task.
— Wikipedia

• Linkback
• Refback
• Trackback
• Pingback

Webmention is the newest of these protocols, and it is also a W3C recommendation.

• https://en.wikipedia.org/wiki/Webmention
• https://www.w3.org/TR/webmention/
• https://indieweb.org/Webmention

If these techniques are of interest to you, maybe you should also check out POSSE.