The only way I know of now (since the feature was disabled) is if that user puts his/her username in the rep comment or some other indication to give away his/her identity. I try to put my username or some shortened version like tt or 2trblsm.
Although since it only shows the last 5 reps/thanks on the list, it is possible to get a rep and not catch it before it gets pushed down the list because others rep you or you get thanks. Thanks do not show you who thanked you in the User CP anymore. You have to go to the post to see who has thanked you.