Source: https://www.reddit.com/r/mturk/comme...or_requesters/

Apparently, if you go to the individual HIT in the GUI and click on the Approved tab, all bonus amounts will be shown with a star under the worker ID of the person who completed the HIT.

Workers: if you need to tell a Requester how to find this because they promised you a bonus that never arrived, use the link above as it has helpful screenshots.