linkback

a method whereby the author of a web page can receive notification when another author links to their website

linkback

Any of several automated or semi-automated mechanisms which notifiy an author of a page on a website when another author links to their page from another web page, anywhere on the Internet. Types of linkbacks include trackback, pingback, webmention, and refback.

(You could send a postcard, but that's more labor intensive and prone to human error.)