Hi Youngjin Kim,
Thank you for your interest in Syncfusion Products.
Please refer the below online UG document to know how to detect whether a new link has been added/removed from diagram.
http://help.syncfusion.com/ug_83/User%20Interface/Windows%20Forms/Diagram/Documents/howtodetectwhetheranewlinkhasbeenaddedremovedfromadiagram.htmPlease let me know if you have any concerns.
Regards,
Pandi Murugan A