by clicking the User CP button in the top toolbar at any page in the forums, or on a profile. It will not give you the name, unless you are a loyal, or a staff member, but it will give you the comment and whether or not it was good or bad.
the whole point of repping is that they dont know or else it would become a fight type thing when someone bad repped you. ill tell you this tho you can be sure who gave you the most recent rep